Hi, cjgodfreyt.
Welcome to SR.
The following thoughts are my opinion only, so take what you like and leave the rest.
It sounds like your spouse has mixed feelings about her mil, maybe some continuing grief about her mother’s death and resentment about your mother’s behavior.?
What are your motives here?
Obviously to protect your child, which I think can be done without too much drama.
Mom doesn’t baby sit. Mom doesn’t walk downstairs with child. Whatever you need to do to ensure your child’s safety and wellbeing.
Do you think that barring your mom from seeing the baby, if that is what you plan to do, will cause her to stop drinking?
Cuz, no.
We stop drinking when we are ready to stop drinking.
If your mom has been drinking to excess for 20 years, nothing you say or do will change that, sorry to say.
As a grandmother, I feel it is better for all if the child or children can have a relationship with their grandparents, unless the grandparents are messed up, toxic, or dangerous.
Do you feel that is the case in your situation?
I am very, very sorry for your situation, and I would like to say again that the above is my opinion only.
You must do what you must do.
Good luck and good thoughts.