Sibling-in-law

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

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.