A sister-in-law (plural sisters-in-law) is the sister of one's spouse, the wife of one's sibling, or sometimes the wife of one's spouse's sibling. Relative to a woman, the latter case (typically her husband's brother's wife; that is, the two women have married brothers) is occasionally called a 'co-sister-in-law', as they are each the sister-in-law of the others husband.
For example, if Jacob and Esau are brothers, Leah being Jacob's wife, and Adah being Esau's wife, then Adah is Jacob's sister-in-law, Leah is Esau's sister-in-law, and Adah and Leah are co-sisters-in-law to one another.
Here's another example: If Bill Weasley and Ginny Weasley are brother and sister, Fleur Delacour being Bill's wife and Harry Potter being Ginny's husband, then Harry is Bill's brother-in-law, Fleur is Ginny's sister-in-law and Harry and Fleur are brother-in-law and sister-in-law to each other.
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