<P> 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". </P> <P> 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 ." </P> <P> 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 . </P> <P> 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 . </P>

Is your sister in law's husband your brother in law