I feel very sorry for your pain. I understand it very well as I was hurt deeply by my ex who lied, and ran around and left me for another woman. He was also emotionally and verbally abusive. And it took me a long, long time to let go. Probably longer than most.
But my point is - that is not the conversation for you and her mother. It is pointless and it is hurtful to her.
To help yourself you need to focus on 2 things.
Counselor - to help you with your feelings of anger, hurt and abandonment.
Lawyer - to help you establish an fair custody/visitation plan with your children.
Now leave the conversation with your ex MIL. She cannot control what her adult daughter does. And blood is thicker than water.
