PALE- Your statement - " They ( your mom & other JWS ) just assume she's not going to meetings for a bit but will " come to their senses " . And that is what's part of the problem here. Your mom and her sissy husband with no backbone are being intrusive into your ex-wife's boundaries and your children, and by extension your boundaries - by trying to " save " your wife by being " supportive " to her and I'm sure part of the motive is to keep JW control over your wife and children.
The other posters here are correct. This is between you and your ex-wife. I would stop getting into it with your JW mom as your mom will manipulate the situation to align herself in a more deeply entrenched way with your ex-wife and child. You need to have a one on one talk personally face to face with your ex-wife to sort this out and attempt to get her on the same page with you- avoiding your mom's interference. Just my 2 cents. Best of luck to you my friend. Been there, experienced this with my JW ex-wife years ago. It sucks man