Siblings-in-law are related by a type of kinship called affinity like all in-law relationships.
[3] 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.
"[4] In Islamic law (Sharia)[5] and Jewish law (halakha),[6] 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.