Is your brothers wife's sister your sister-in-law?

Your sister-in-law is either the wife of your brother or the sister of your spouse. Your brother-in-law is either the husband of your sister or the brother of your spouse.

So your brother-in-law's wife is either your sister or is not related to you.

Some people incorrectly refer to a brother-in-law's wife as a sister-in-law.