Hey sweeetie. Happpy Mother's day - I hope your son was extra affectionate and helpful to his mom today!
I'm no expert on legal issues and custody battles, but my gut says that you have every right to do whatever is best for you and your son. It sure sounds like the relationship is unhealthy and J shouldn't have to be exposed to it. If your exMIL won't even let her in the house, it really must be a bad situation. Hugs...Hope things calm down for you soon.