David and Jonathan, sworn friends and confidants, became brothers-in-law when David married Jonathan's sister Michal.[1]

One's sibling-in-law is the sibling of one's spouse or the spouse of one's sibling. By gender, this is specified as brother-in-law for the brother of one's spouse or the husband of one's sibling, and sister-in-law for the sister of one's spouse or the wife of one's sibling.[2]

Just like other affines, or "in-laws", siblings-in-law are related by a type of kinship called affinity. A more distant type of relation is that of a co-sibling-in-law; that is, one's spouse's sibling's spouse or one's sibling's spouse's sibling. 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."[3]

In Islamic law (shariʿa)[4] and Jewish law (halakhah)[5] 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.[6]

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

Look up sibling-in-law in Wiktionary, the free dictionary.


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

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