Noah Webster's Dictionary (n.) The sister of one's husband or wife; also, the wife of one's brother; sometimes, the wife of one's husband's or wife's brother.

Multi-Version Concordance Sister-in-law (1 Occurrence) Ruth 1:15 She said, "Behold, your sister-in-law has gone back to her people, and to her god. Follow your sister-in-law." (WEB JPS ASV BBE DBY WBS YLT NAS RSV NIV)