What Do I Call My Brother-in-Law's Sister?
A person's brother-in-law's sister is potentially that person's wife, sister-in-law or no relation whatsoever. What one calls his brother-in-law's sister depends on how that brother-in-law is related and which sister it is.
A male becomes a brother-in-law to someone in one of three ways. He is the brother of one's spouse, the husband of one's sister or the husband of one's spouse's sister. In the first case, the sister is either a spouse or a sister-in-law. In the second and third scenarios, the sister is no relation at all. She is called either by her first name or a title and her last name depending on the closeness of the relationship.