Your sister-in-law may be the wife of your brother. If so, her daughter is your brother's daughter and your niece. You are her aunt or uncle. If your sister-in-law has a daughter fro a previous...
Your sister-in-law's husband may be your brother. If so then the brother-in-law is either your husband, in which case the daughter is your daughter; or the brother-in-law is your brother, so the...