NON GENETIC RELATIONSHIPS (All definitions are given in italics)

Apart from religious guardians such as godparents and legal guardians who act in loco parentis, there are three major causes of non-genetic family relationships: marriage, extra-marital partnerships and adoption. Relationships by marriage include: in-laws, step relatives and spouses.


These are subdivided into two types:

(a) Your spouse's relatives and

(b) Your relatives' spouses.

Type (a)Type (b)
Brother-in-law (Spouse's brother) Brother-in-law (Sister's husband)
Sister-in-law (Spouse's sister) Sister-in-law (Brother's wife)
Uncle-in-law (Spouse's uncle) Uncle-in-law (Aunt's husband)
Aunt-in-law (Spouse's aunt) Aunt-in-law (Uncle's wife)
Nephew-in-law (Spouse's nephew) Nephew-in-law (Niece's husband)
Niece-in-law (Spouse's niece) Niece-in-law (Nephew's wife)
Father-in-law (Spouse's father) Son-in-law (Daughter's husband)
Mother-in-law (Spouse's mother) Daughter-in-law (Son's wife)
Grandfather-in-law (Spouse's grandfather) Grandson-in-law (Granddaughter's husband)
Grandmother-in-law (Spouse's grandmother) Granddaughter-in-law (Grandson's wife)

Exceptions to the above rule occur when there has been more than one marriage. Non-blood relatives who have been carried over from a previous marriage are known as step relatives. (See later). Only one degree of separation is allowed for in-laws. This means that your wife's brother's wife is not your sister-in-law, she is your wife's sister-in-law.

In all two-way relationships it is sometimes convenient for one member, who is the primary focus of attention, to be referred to as the proband and the other to take a secondary role. This means that the relationship is looked at from the proband's point of view. For example, in an uncle - niece relationship, if the proband is the uncle, then the relationship of the secondary person to him is niece. But, if the niece is the proband then the other person is her uncle. This may seem a trivial point but it is sometimes important to know whose stance we are taking when considering two people's relationship. The same rules apply to non-genetic relationships. In the following diagrams the proband is printed in red and the other person in green.

It is interesting to consider why there are two types of the first six in-law relationships but only one type of each of the last eight. What the first six have in common is that if they had been real genetic relationships, they would have been classed as collaterals whereas the last eight have a direct relationship connotation. This means that according to the above definitions for type (a) and type (b) in-law relationships, a type (b) father-in-law would be your mother's husband and a type (a) son-in-law would be your spouse's son. Your mother's husband and your spouse's son are not true in-laws since the former is either your real father or your stepfather from your mother's second marriage and the latter is either your own son or your stepson from your spouse's first marriage. The same reasoning applies to the remaining three pairs of in-laws giving: father's wife and spouse's daughter; grandmother's husband and spouse's grandson; grandfather's wife and spouse's granddaughter.

Since we are only dealing with non-genetic relationships, it is unnecessary and would be pedantic, in this context to distinguish between the three types of sibs and uncles-nieces etc, (i.e. first degree, full and half). For the same reason uncles - nephews and grandparents - grandchildren are the widest relationships considered. Cousins are too distant to be regarded as in-laws or step relatives.

In the following diagrams, to show when two people are either married or in a stable relationship, a 'token' child is inserted, where necessary, to link them together in an otherwise non-genetic situation. Although children may not actually be present in every case, their inclusion is the only way (in arrow diagrams) to indicate the true marital status of all the couples. This device does not interfere with the accurate portrayal of each non-genetic family relationship. The final point to note is that the proband can be of either sex . Only one sex is shown in the diagrams for convenience. Figs.58 to 67 are graphic illustrations of the 14 different kinds of in-laws:

Figure 58 Brother-in-law

Figure 59 Sister-in-law

It can be seen that for the first two kinds of in-law only, type (a) can be converted into type (b) and vice versa simply by switching the role of proband to the other person in the relationship. This is because the proband and the in-law are in the same generation. For all the other in-laws, reversing the proband reverses the direction of the relationship; e.g. Father-in-law becomes son-in-law (or daughter-in-law).

Figure 60 Uncle-in-law

Figure 61 Aunt-in-law

Figure 62 Nephew-in-law

Figure 63 Niece-in-law

Figure 64 Father-in-law and Son-in-law

Figure 65 Mother-in-law and Daughter-in-law

(Mother-in-law - Son-in-law) and (Father-in-law - Daughter-in-law) are the only two pairs of in-law relatives who are still not allowed to marry if either or both of their original spouses are still alive. This law was challenged recently by two such people who wanted to marry.

Figure 66 Grandfather-in-law and Grandson-in-law

Figure 67 Grandmother-in-law and Granddaughter-in-law

All the foregoing in-law relationships are only applicable for the duration of the current marriages. After marriages have been dissolved they become ex in-laws. Similarly, if a partner dies and the person remarries, he or she acquires a new set of in-laws.

Step Relatives

When a person remarries following the death of his/her original spouse (or after a divorce), then the new spouse becomes stepparent to any children from the previous marriage . The same situation occurs when non-married partners with children form new relationships. However, if both partners in the second marriage carried children over from previous marriages, then each of them would have step relationships with the children (and grandchildren) from the first two marriages. (See Figs. 72 to 75).

With stepbrothers and stepsisters, the children from both original families have the same step relationship to each other; it simply depends on who is considered to be the proband in each case. (See Fig. 76). After second marriages, problems arise in deciding whether the children will reside with their father or stepfather on the one hand or with their mother or stepmother on the other. However, it is possible for all the children from both first marriages to live with the partners of the second marriage in one big family. These situations, where three groups of children are involved, are unusual for a different reason. The children from the second marriage are half sibs to both sets of children from the first two marriages but the separate sets of children from the first two marriages are not related to each other and can legally marry. These topics were considered in the original monograph. (See here and here). Please note that D and E in Figs. 68 and 69 are half sibs, not step relatives.

In current usage, the names of step relatives are usually written as a single word without a break or hyphen (e.g. stepfather). Most dictionaries restrict the definition of step relatives to relationships between immediate family members following the remarriage of a parent. These are stepparents, stepchildren, stepbrothers and stepsisters. However, there is no logical reason why relationships following the remarriage of grandparents should not be included. For example, I have always regarded my grandmother's second husband as my stepgrandfather.

Prior to the 20th century the only step relatives who were allowed to marry in the UK were stepuncles - stepnieces, stepaunts - stepnephews and stepbrothers - stepsisters. The following definitions, supplemented by diagrams, are an attempt to rationalise and describe all the 14 possible types of step relatives. Unless otherwise stated, the word 'spouse' refers to the current spouse.

Figure 68 Stepfather and Stepmother

Parent's current spouse who is not one's own parent.

Figure 69 Stepson and Stepdaughter

Current spouse's child by a previous partner.

Figure 70 Stepgrandfather and Stepgrandmother

Grandparent's current spouse who is not one's own grandparent.

Figure 71 Stepgrandson and Stepgranddaughter

Current spouse's grandchild by previous partner.

Figure 72 Stepfathers and Stepmothers (Two Previous Marriages)

Please note that the stepparent's previous (first) spouse in each case is not a stepparent to the children from the other first marriage. He or she's only relationship to them is that of stepparent's ex spouse. Neither is the stepparent's ex spouse a stepparent to the children from the second (current) marriage; only the former spouse of one of their parents.

Figure 73 Stepsons and Stepdaughters (Two Previous Marriages)

In this case the stepchild's spouse (not shown) is not a stepchild of the stepparent. This is one degree of separation too far.

Figure 74 Stepgrandfathers and Stepgrandmothers (Two Previous Marriages)

Figure 75 Stepgrandsons and Stepgranddaughters (Two Previous Marriages)

Figure 76 Stepbrother and Stepsister

Child of one's stepparent by a previous spouse (or partner).

With serial marriages or when the same person marries several times, the situation is different. (See Figs. 77 and 78). To save space in the next two diagrams, not all the sex combinations are shown, and both members of each typical two-way relationship (printed in red) are named in the same diagram.

Figure 77 Serial Marriages

D♀ is stepmother to F♀ because C and D are currently married, but C♂ is no longer stepfather to E♂ because B and C are divorced. He could only be styled as E♂'s former stepfather. Also, A♂ has no relationship to F♀ except that of her mother's former husband. E and F are half sibs and also F and G, but E and G are unrelated; not even stepbrothers or former stepbrothers.

Thus, it is only the current spouse of your parent who can have the title of stepparent to you.

Figure 78 Serial Marriages to the Same Person

In this situation D♀ is stepmother to both E♂ and F♀ but C♀ is only former stepmother to E♂. This is because the current marriage is between A♂ and D♀. E, F and G are all half sibs of each other.

When your spouse has previously been married several times, you are stepparent to all the children from his/her earlier marriages.

The next four step relatives: stepuncles, stepaunts, stepnephews and stepnieces; are less clearly defined than the others. This is because they can arise from either the remarriage of parents or the remarriage of grandparents. This, together with their remoteness from the nuclear family, is probably the reason why they are not usually included in lists of step relatives. However, to bring the numbers of step relatives in line with in-laws, both forms are depicted in Figs. 79 and 80.

Figure 79 Stepuncle and Stepaunt

Remarriage of Parents

Parent's second (current) spouse's brother or sister. (i.e. Your stepparent's brother or sister).

Remarriage of Grandparents

Grandparent's second (current) spouse's son or daughter. (i.e. Child of one's stepgrandparent by previous partner).

Figure 80 Stepnephew and Stepniece

Remarriage of Parents

Remarriage of Grandparents

Parent's second (current) spouse's grandchild. (i.e. Your stepparent's grandchild).

Marriage and Inheritance Laws for Step Relatives and In-laws

In the UK before 1907, none of the above in-law pairs of relatives and only some of the step relative pairs were allowed to marry. All in-laws and step relatives can now marry with certain restrictions:

(1) You can only marry your former stepparent or stepgrandparent (of the opposite sex) if you are over 21 and, under the age of 18, were not brought up in the same household as your step relative. If your parent has been married several times this rule applies to all your former stepparents and stepgrandparents. Conversely, it also applies when a stepparent has remarried and has more than one set of former stepchildren.

(2) You can only marry your former mother-in-law or former father-in-law if you are over 21 and the spouses of both you and your former parent-in-law are both deceased. If either of the two spouses are still alive, because of divorce, then the marriage is illegal. If either of you have been married several times then this law applies to all your former mothers- and fathers-in-law and former sons- and daughters-in-law.

The situation with intestate wills is that in-laws and step relatives are not included in the list of beneficiaries.

Marriages Between In-laws and Between Step-relatives

It was shown here and here in the main monograph that marriages between in-laws can lead to enhanced half sibs relationships. In particular, marrying your brother- or sister-in-law can produce a special form, i.e., 3/4 sibs (R = 0.375), Three-quarter sib relationships can also result from the following marriages: father –in-law with daughter-in-law, mother –in-law with son-in-law, stepfather with stepdaughter, and stepmother with stepson. (Figs 81, 82 and 85). Even though there is no inbreeding involved, these marriages between in-laws and between step relatives can produce some vary strange relationships between family members. The following section analyses some of there unusual situations (Figs 81 to 86):

Figure 81 Marriage to Father-in-law

D♂ and E♂ are half brothers (RDE = 0.25)
E♂ and F♂ are half brothers and E♂ is also F♂'s half uncle,(REF = 0.375)
E♂'s father (A♂) is grandfather to E♂'s half brother. (RAE = 0.5) and (RAF = 0.25)

Figure 82 Marriage to Sister-in-law

F♂ and G♂ are half brothers,they are also single first cousins(RFG = 0.375)

Figure 83 Marriage to Niece-in-law

G♀ and H♂ are single first cousins (RGH = 0.125)
H♂ and I♂ are hald brothers and also first cousins once removed (RHI = 0.3125)
H♂'s grandfather (A♂) is great grandfather to H♂'s half brother (I♂).(RAH = 0.125) and (RAI = 0.125)

Figure 84 Marriage to Grandfather-in-law (Husband's Grandfather)

G♂'s father (A♂) is great grandfather to G♂'s half brother (H♂). (RAG = 0.5) and (RAH = 0.125)
Although G♂ is younger than F♂, he is his half uncle (RGF = 0.125)
G♂ is H♂'s half brother and also H♂'s half great uncle.(RGH = 0.3125)

Figure 85 Marriage to Stepmother

D♂ and E♀ are half sibs (RDE = 0.25)
F♂'s half sister (E♀) is also F♂'s half aunt (REF = 0.375)
E♀ is also half sister to her half brother (F♂'s) father (D♂). (RDF = 0.5)

Figure 86 Marriage to Stepsister

E♂ and F♂ are half sibs (REF = 0.25)
F♂ and G♀ are also half sibs (RFG = 0.25)
But E♂ and G♀ have no genetic relationship(R = 0)
H♂ is F♂'s double half nephew (RHR = 0.25)

Spouses and Partners

Spouse is the general term for husband or wife and means that the couple have undergone an official marriage ceremony. In Western countries it is illegal to have more than one spouse but some Eastern and African countries allow polygamy where more than one spouse is permitted. This usually takes the form of polygyny where a man has more than one wife. Polyandry, where a woman has more than one husband is less common. It used to be practiced in Tibet before that country was annexed by China. With polyandry, there may be some uncertainty about the exact male parentage and sibling relationships of the children. This is less of a problem with polygyny since it is usually easier to identify a child's mother than its father.

Since about 1960, the practice of having children outside marriage has gradually become accepted in Western society, where it is no longer regarded as abnormal or immoral. This has to be recognised in any discussion about relationships and the word partners is now sometimes used to cover married couples as well as unmarried ones to avoid discrimination. Similarly, the word, illegitimate, is no longer used as a derogatory label. For unmarried people living together in a stable relationship, there is no reason why the terms in-laws and step relatives should not be used in the same sense as for married couples. The only difference is that the word partner replaces spouse and although in-law is not really accurate, the alternative out-law has other connotations!

These changes have not affected the meaning of all the true genetic relationships such as: father, mother, son, daughter, brother and sister etc. which are still based on the same fundamental biological links. In all the diagrams, these are shown as true genetic pathways connecting family members.


The relationship between adoptive parents and their legally adopted children is straightforward and does not need elaboration. The rules for marriage within these non-genetic families are quite clear:

1.Adoptive parents can not marry their legally adopted children.

2. Children who are adopted by the same adoptive parents can legally marry each other provided they are not on the forbidden marriage list. This would normally only include: full brothers and sisters, half brothers and sisters, uncles and nieces, aunts and nephews, half uncles and half nieces and half aunts and half nephews. The reason for including the last four is that although uncles and nieces etc. are from different generations, they can be of similar ages. (See here). The same rules apply if natural and adopted children occur in the same family. The natural children can only marry the adopted ones if they are not closely genetically related.

3. Children who are adopted by different adoptive parents are also unable to marry if they are genetically closely related and on the above forbidden list.

An adopted child has the same legal rights of inheritance, in an intestate situation, as the adoptive parents' other natural children. However, adoption removes any rights the adoptee may have had to his or her natural parents' estate. (Elmhirst, 2007).

Quantitative Measures and Coding of Non-genetic Family Relationships

By definition, the Coefficient of Relationship (R) between non-genetic family relatives is normally zero. However, if there has been any legal (or illegal) inbreeding, this may not be the case. Also, if adoptive parents adopt children of their own blood relatives, R will have a measurable value. In the section on the coding of relationships in the main monograph here examples are given of in-laws, step relatives, spouses, adoptive parents and adopted children, where there are double relationships combining both genetic and non-genetic components.

Finally, coding for all types of in-laws, step relatives, spouses and relatives through adoption is given in the main monograph here. In the case of stable partnerships outside marriage, a new code is now required. I suggest the use of Pm and Pf when referring to male and female committed partners who are not currently married.

In the foregoing discussions I have deliberately avoided gay partnerships and marriage. With the institution of same-sex marriage, this now has to be addressed. Any new codes have to be in the same format as the previous ones, which are all one-way; i.e. as viewed from the other member (the proband) of a two-way relationship. To accommodate this restriction and also distinguish them from the codes for conventional partnerships I suggest the following:

MarriageCommitted Partnerships
Same sexGMmGMfGPmGPf
Opposite sexHWPmPf