Karen's advice is good. Be upfront.

My stepmom attended my wedding. I had some major drama at my wedding because I didn't discuss that with my mom ahead of time. We still fight about my wedding to this day. I think she wouldn't have been so shocked and would have felt more comfortable if I'd given her a heads up. If I'd warned her to be on her best behavior, perhaps she wouldn't have flipped out.

I love your dress!