A person's sister-in-law is his or her brother's wife, or spouse's sister. The term is often misapplied to the wife of a person's brother-in-law; it's true only in the case that this brother-in-law is the person's spouse's sister's husband, which makes him a brother-in-law by marriage. Therefore, it would be correct to say that the wife of a person's brother-in-law by marriage is a sister-in-law, but a more simple way to refer to this sister-in-law is as a person's spouse's sister. If a person's… (More on Sister-in-law)