<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>

What to call your brother in law's wife