Be a sly fox and stay out of it. Set expectations/boundaries between how you both interact with each other and nip it in the bud if she does it to you. She'll learn!

My husband's ex is cray cray, but I stay out of their drama and provide guidance when asked by step-DD.

Gosh, this is a tough one. It depends on the comfort level in the stepmom/child relationship I guess. You don't want her to feel like you are tattling on her. And it is her mother's responsibility.

If's she's respectful to you and your DH that's really all you can demand in your home I suppose.