It's natural that your mom wants your stepdad to be part of your life. It's part of her trying to bring him into the family. At the same time, it would have been nice if she had asked you first if he could come along. Since she didn't, you have to make it plain. When you have a calm moment alone with her, you might say something like this:
"Mom, I understand that you wanted (stepdad) to come and see the painting I did. But painting to me is something very private. I'm very uncomfortable with my work being in public and I didn't want a big fuss made over it. I was hoping that just you and I would go and see it. In the future, I'd like you to ask me if I'm comfortable with (stepdad) going somewhere with us."
If that goes over well, you might consider asking her if she knows where your other paintings are. Even though you know she took them, do not say, "Where are my paintings that you took?" Just say something like, "By the way, those paintings that you mentioned at the show--I can't seem to find them. Do you have any idea where they might be?" That way, you don't sound like you're accusing her and hopefully she won't get defensive.
I hope this helps. If it doesn't, I apologize. Good luck!