A sister-in-law is one's brother's wife, or one's spouse's sister. One's spouse's brother's wife is also considered a sister-in-law. The plural of this term is "sisters-in-law".
Example: Gwyneth and Rob are married. Rob has a brother, Neil, and so Gwyneth is Neil's sister-in-law. Neil will marry Claire, who will then become Rob's sister-in-law. Furthermore, Gwyneth and Claire will become sisters-in-law to one another.