After yesterday, I think I understand my role a bit more.
Dump the kids on them and leave. Do everything that is hard to get to with the kids, leave the house, complete a project, hit the gym, go sit in your car, anything but stay home. MIL watched the kid while we were at work. Everyone survived, everyone was in a good mood, she left for dinner with my sister in law. I barely interacted with her and everyone was happy. All of the conflict comes from when we are stumbling over each other, I'm being peppered with stupid questions, or I'm being overly critical of her every move.
This is the way.
Now I wish she was here one more day. There's a spot around the corner that should be cooking (probably packed though) this afternoon.