People call me sister-in-law What should I call him?

People call me sister-in-law I should call him brother-in-law or brother. Because the person who calls you sister-in-law is definitely younger than you, and boys call him brother-in-law or brother.

You can call them by their first names because you are older than them and should be called sister-in-law. It's impolite not to call you by your first name.