Ideas for stopping ex and mil talking in front of children

[Lmls]

My ex and mother-in-law have discussed his and mine relationship in front of our children on several occasions. My daughter, 15 has reported back, upset. After the first time I spoke directly with mil and she said it didn't happen. I avoided arguing by saying whilst that was her perception it was not my daughters and to please show more care in the future as I knew that she did love my daughter and cared about her feelings. It happened again at Christmas when my young son came home saying that we were the sadistic side of the family, and yet again since. I have told me daughter when it happens to simply say that it isn't appropriate and to leave the room. In fairness I also said that if the same thing ever happened with any of my relations to do the same with my backing- her comment was that she knew that wouldn't happen within my family.

I have also raised the issue with my ex, pointing out how it upsets the children and suggesting a way to build his relationship back up, (which he says he wants to do) with his daughter in particularly, is to stop doing this and to tell his mother the same if she starts.  He agreed but recently on a holiday my three children, son 16, daughter15 and son 6 were in a bedroom whilst their grandparents and father discussed us and them negatively. My daughter has described me making her go on holiday with them as a sick joke and hasn't seen her grandparents since.

At least my older two can talk to me about it if they wish but I am concerned for the impact on my younger son as well who spends more time alone with them than the other two.

I think my ex has inherited his BPD and ND traits from his mother so maybe no real hope for change. Any ideas?