Just wanted to add something about legally caring for a parent.
In my case, my mother was a widow (no living spouse). I needed legal appointment to handle her financial and medical responsibilities.
In your MIL's case, there is a living spouse. Please check with an attorney and her doctor.
This is based on my experience with caring for a parent. I have not taken care of an alcoholic parent.
As we say in Alanon, "Take what you like, and leave the rest."
This decision is between you, your wife, MIL, her spouse, any other siblings and your HP (higher power). May you find a solution that brings you peace.