Ok I too have a nutty MIL. H and I always discussed everything regarding her and S but we were also on the same page with it for a long time. These last couple of years, H has sort of decided S is old enough to make his own choices and I think he has wished S would have wanted more contact but understood why he didn't.
Very recently, I had to make my choices as well. I had to decide what I was going to allow for my S in this situation and what I thought was the best for him, regardless of what H or S thought. I never wanted to be one of those mothers who kept her child from his grandparents but as I see that she has not changed her behaviors in any way and H is in La La Land, I cannot leave S to flounder. I have only spoken of my boundaries with H and S but they are in place now. You may really have to set boundaries with H and maybe with her if he will not tell her.
They don't really know how to deal with their parents and that is part of the problem in MLC. Just be prepared for the anger that will come if you set boundaries because you are then being the "bad" guy again. I did it because I had to protect my S. I do not want him so damaged that he will go through what H is going through now (of course I couldn't put it to H that was). In the past, I went behind and cleaned up all of her messes. I can't do that anymore because it is not good for me. So I have had to take an offensive stance. Can't protect H though. Wish I could. But I have told him that is his thing to deal with.
If you have reasons other than it will make it harder for H to run around, then do what you feel is right. If it is simply to stop him, you might want to rethink it a little. H will just find another way to do what he wants, until he decides it isn't what he wants anymore. The thing I have tried so hard to remember during this is that they are like children or teenagers, and if you think back to when you were a kid, I'm sure you will remember that the more your parents told you NOT to do something, the harder you worked, the sneakier you became, to find a way to do it. It is human nature. Good luck, this was something I really struggled with.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox
