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Suppose that two parents have a child named Sam before divorcing. Sam's father then marries another woman and has a daughter, Daisy, while Sam's mother marries another man and has a son, Matthew.

Because Sam shares a parent with each of them, Matthew and Daisy are both his half siblings. However, Matthew and Daisy are not siblings in any way, for they do not share any parents nor do they share stepparents.

Could Matthew and Daisy legally get married and have children if they are living in a state where incest is illegal? Would this count as incest at all?

I would also like to know if there is a term for Matthew and Daisy's relation to each other. (Quarter siblings? Some type of cousin?)

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  • Is there any state that permits incestuous marriage? I doubt it. There must be differences in the degree of relationship that constitute incestuous marriage, but I suspect that having parents that were one married to each other or having a mutual half sibling would not constitute incest anywhere.
    – phoog
    Dec 31 '17 at 10:26
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    @phoog While every society has some form of incest prohibition, there are definitely different definitions of incestuous marriage that are used in different societies and U.S. states, so the fringe cases definitely could vary. For example, there is a big divide between U.S. states concerning whether first cousin marriage is barred. Peculiarities of wording with unintended effects could definitely impact this case.
    – ohwilleke
    Dec 31 '17 at 20:52
  • @ohwilleke of course. I was principally reacting to the clause "if they are living in a state where incest is illegal." There is also of course the possibility of a difference in definition between incestuous marriage and an incestuous sexual act, which is what I normally understand the noun "incest" to denote, but that doesn't seem likely to have a bearing on this question.
    – phoog
    Dec 31 '17 at 21:28
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    @ohwilleke Some states forbid marriage between uncle and niece, and between aunt and nephew. Others forbid marriage between uncle or aunt and niece or nephew. These laws are probably old, and the lawmakers saw no difference between them. Suddenly, in the 21st century, that makes a difference if uncle and nephew or aunt and niece want to get married.
    – gnasher729
    Mar 22 '20 at 16:44
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    @gnasher729 Some states prohibit based upon "degree of relation" according to a standard civil law system.
    – ohwilleke
    Mar 23 '20 at 1:51
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Assuming that neither one of Matthew's parents are blood relatives of either of Daisy's parents, then they do not share a blood relation that would forbid their marriage under any law or standard of incest of which I have heard.

(As for describing the relationship of Matthew and Daisy: The best English term is step-siblings, even though the order of parental relationships that produced them may not be the most common sequence that produces step-siblings.)

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Biologically the marriage is absolutely fine.

Legally, you’d have to check the exact laws of the country in question. A country could have laws that are from a biological point of view unreasonable. I wouldn’t be surprised if marriage between adopted children was illegal, for example.

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    It is also worth noting that biology isn't the only legitimate reason to have an incest prohibition. For example, step-parent to step-child marriage would frequently involve a breach of a position of trust and might be barred out of that non-biological concern.
    – ohwilleke
    Dec 31 '17 at 21:32
  • That’s a good point. I had always thought it’s because when these laws were created, you couldn’t know that “stepdad” wasn’t in reality the real biological father. I think the US had laws treating cousins differently depending how the parents were related, because some combinations had the possibility that there was some affair making cousins much more closely related (worst case cousins with sisters as mothers, where one husband might have an affair with the other sister).
    – gnasher729
    Mar 30 '20 at 8:48
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The term you're looking for is "cross-siblings" and considering they're not biologically related at all, there is no reason why they shouldn't be able to marry each other.

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    No biological reason. Laws can be strange, or even stupid or unfair.
    – gnasher729
    Mar 23 '20 at 22:47
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This is definitely NOT considered incest if Sam's half-brother and half-sister grew up in separate households. The problem arises that when Sam's half-siblings marry and have children, and one day they discover that their uncle is the brother of each of their parents. He's literally on "both sides" of his nieces & nephews' family tree. Which side of the aisle does uncle Sam sit on when his nieces and nephews get married? The bride's side or the groom's side?

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