Sister-in-law

A sister by marriage.

A person's sister-in-law is his/her brother's wife, or spouse's sister.

There is some debate over whether the wife of a person's brother-in-law is "sister-in-law" or "sister-in-law by marriage", however, most legal definitions indicate that a sibling through marriage is a "brother-in-law" or "sister-in-law".

The plural form is "sisters-in-law".