(uncommon) One's brother-in-law or sister-in-law's brother; that is, one's sibling's spouse's brother (one's sister's husband's brother or one's brother's wife's brother). (in the plural) Men whose brother and sister are married to each other.
One's husband's brother's wife; or more generally one's spouse's sister-in-law, the sister of one spouse in relation to the siblings of the other spouse. One's brother-in-law or sister-in-law's sister; that is, one's sibling's spouse's sister (one's sister's husband's sister or one's brother's wife's sister).
David and Jonathan, sworn friends and confidants, became brothers-in-law when David married Jonathan's sister Michal. Sibling-in-law is a word phrase referring to the sibling of one's spouse, or the spouse of one's sibling, or (more rarely) taken one step further by referring to the spouse of one's spouse's sibling.