A sibling-in-law is the spouse of your sibling, or the sibling of your spouse, or the person who is married to the sibling of your spouse. More commonly a sibling-in-law is referred to as a brother-in-law for a male sibling-in-law, and a sister-in-law for a female one.
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). (in the plural).
(uncommon) One's wife's sister's husband; or more generally one's spouse's brother-in-law, the brother of one spouse in relation to the siblings of the other spouse.