MIL

I’d just be straight up. If she’s rude and disrespectful to you give her the taste of her own medicine and don’t sugar coat anything x

Start by saying that you want to do this from respect, but that she will have from you what she gives you, if she is rude to you do not let yourself, defend yourself, because although she is the mother of your husband, you as a mother your duty is to take care of your family, your children.

Why is he not having the conversation with you being involved but him leading it?

Tell her straight zx

I think your husband should be having that conversation with his mum. It shows her that you are united and he isn't prepared to have the mother of his child spoken to in a disrespectful way

Honestly I from experience I think that having a conversation like you are thinking to wont work and they will twist what you say and make you sound petty, instead I would call out behaviors at the exact time they happen, just say calmly well that's very rude and inappropriate. I don't appreciate how you're talking to me thank you. If you come up with things that they've said in the past in a conversation they will likely deny it or make you out to be in the wrong