I'm not married, but my SIL calls my mom by her first name, and my other SIL just says "hi" and never really uses a name. If I were you, I would do as suggested before, ask them what they would like to be called. In the past, I have called my boyfriends mom's "mom" Just as Johanna said, it seems to bring you closer. The two women that I call/ called "mom" I was much closer to. Good luck!