I dislike your mil a lot. I feel upsrt reading what she is saying. It must be five thousand times worse for you.

You need neither reason nor excuse. You are Mama and they play by your rules or not at all. Perspnally, I would (and have) just said no when she starts up, then hang up. Don't let her push or ask why. Just "no" and hang up. Not like she hadn't been told no before, she's just too selfish to consider your feelings valid. And to laugh at them! Aaagh.

No and hang up. Not even goodbye. Then don't pick up the phone when she calls back.