A sister-in-law's the sister of one's spouse. The wife of one's sibling and the wife of one's spouse's sibling are also considered a sister-in-law. The plural of this term's "sisters-in-law". Example: Rob and Neil are brothers. Gwyneth's Rob's wife. Claire's Neil's wife. Therefore, Gwyneth's Neil's sister-in-law, Claire's Rob's sister-in-law and Gwyneth and Claire are sisters-in-law to one another.… (More on Sister-in-law)