One’s sibling-in-law is one’s spouse’s sibling, or one’s sibling’s spouse. By gender, this is specified as brother-in-law for one’s spouse’s brother, one’s sibling’s husband, or one’s spouse’s sibling’s husband, and sister-in-law for the one’s spouse’s sister, one’s sibling’s wife, or one’s spouse’s sibling’s wife.
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.”
In Islamic law (shariʿa) and Jewish law (halakhah) 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.
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.