B.H.
Hello,
I am also a stepmom. I would say if she is sneaking them that the only way to stop her is to not have the food in the house when she is there. Start with this. Then in a few months or so have a couple things in the house, but don't tell her. Then when you think she has stopped the sneaking you can let her know, that she can have a special snack sometimes but she must ask first. Hope this helps, let me know if you have any other questions.