A brother-in-law is one's sister's husband, or one's spouse's brother. One's spouse's sister's husband is also considered a brother-in-law. The plural of this term is "brothers-in-law".

Example: Rob and Gwen are married. Gwen has a Sister, Claire, and so Rob is Claire's brother-in-law. Claire will marry Neil, who will then become Gwen's Brother-in-law. Furthermore, Rob and Neil will become brothers-in-law to one another.

At least one source uses "sibling" instead of "sister", allowing for the fact that one's brother may be married to a man.


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