Sibling-in-law includes the brother-in-law and the sister-in-law. It refers to the sibling of one's spouse, or the spouse of one's sibling, or (more rarely)[2] taken one step further by referring to the spouse of one's spouse's sibling. All of these are relations which do not relate to the person directly by blood.[3] Alternatively, the spouse of your spouse's sibling may be called a co-sibling-in-law[4], or co-sibling.


Biblical characters David and Jonathan became brothers-in-law when David married Jonathan's sister Michal.[1]

More commonly this is referred to by the gendered terms: brother-in-law, or co-brother-in-law for a male non-blood sibling, and sister-in-law or co-sister-in-law for a female non-blood sibling.[3]

Just like other affines, or "in-laws", siblings-in-law are related by a type of kinship called affinity. Just like the children of one's siblings, the children of one's siblings-in-law are called simply nieces and nephews – if necessary, specified whether "by marriage", as opposed to "by blood" or "by adoption".

One study, examining the issue of envy in the triadic system of sibling, sibling-in-law and spouse, concluded that "The sibling-in-law relationship shared similarities with both spousal and sibling relationships" and that "Relational closeness and satisfaction for all relationships in the triad were correlated."[5]

In Islamic law (shariʿa)[6] and Jewish law (halakhah)[7] sexual relations between siblings-in-law are prohibited as incestuous, unless the spouse is no longer married. Conversely, in Judaism there was the custom of yibbum, whereby a man had a non-obligatory duty to wed his deceased brother's childless widow so she might have progeny by him.[8]

If one pair of siblings is married to another pair of siblings, the siblings-in-law are thus doubly related, each of the four both through one's spouse and through one's sibling, while the children of the two couples are double cousins.

See also


  1. 1 Samuel 18:20–27.
  2. OED entry for 'brother-in-law' describes this as 'uncommon':
  3. Cambridge Dictionaries Online. "Family: non-blood relations".
  4. Wiktionary, 'co-sibling-in-law':
  5. Yoshimura, C.G (2010). "The experience and communication of envy among siblings, siblings-in-law, and spouses". Journal of Social and Personal Relationships.
  6. "Forbidden...that you should marry two sisters at one time"[Quran 4:23 (Translated by]
  7. Leviticus 18:16, 18:18.
  8. Deuteronomy 25:5–10.
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