What would I call my wife's sister?

Answer 1

You would call your wife's sister your sister-in-law. If your wife would have a brother he would be your brother-in-law. It is the same thing for a husband's siblings. Your wife's mother would be your mother-in-law. Her father would be father-in-law.

Answer 2

She would be your "Sister in Law" and you would call her by her given name.