MIL is co-depenendent like most of us here.
I cannot imagine how much time and energy, this takes out of you. What would be the focal point of your life, if all this drama with the ex and his mom were not?
You alone control how much drama you allow in your life and that of your children.
This "we're family" thing is an excuse most of us have used to keep the drama going instead of taking responsibility for ourselves and outcomes.
I mean this as someone who has been there/done that.