I am divorced one year, separated for 3. It was really hard for me, I had a really hard time detaching. I wanted validation. I'll never get that, so I had to leave it. I wanted to be right.
For now, just leave things alone, do not keep talking to mil. It will just keep you involved. Just take care of your children, anyway that you can. Don't make his problems, "your problems". You can fix your own, but not someone else's.
I still wish I could "fix" the problem, but I can't.