yummy I went through this sort of treatment from my ex-DIL who made it her business to create a distance between me and my son. For twelve years I said nothing, offered her support and sat there passively while she criticised him for never being at home, being under her feet, getting drunk when they had a party, being a miserable so-and-so for staying sober when they had a party, not giving her enough money to spend on redecorating four months after they redecorated, buying too many things for her and running up a large credit card bill and so on....nothing ever pleased her. My son would stay out of the way when I visited, claiming he had jobs to do.
The whole family would tread on eggshells around her, and expressed our sorrow about the distancing of my son, their elder brother. He attempted to leave her a couple of times but went back because of their son and said he would wait until his son was 18. but then carried on being supportive to her and distant with us. A year ago, he got to the stage where he couldn't take any more and left her. Her behaviour amounted to domestic abuse over many years. We immediately grew closer - in fact he's staying with me at the moment till he finds another suitable house - he had been living 500 yards away so he could be near his son, but her continual attempts to cause trouble put paid to that.
What I'm leading up to is that your son has a baseline from which he has grown and developed his own adult relationships, and he, like you, will feel this distorted way of relating to the people he cares about, but probably be searching for a way to manage it without one side or the other creating merry hell. You are the easier one to manage - she'll be a nightmare. He won't feel any different about you, but just be hoping things will die down. He won't want a repeat of the incident that resulted in this situation, so needs to keep her away from your other son. Is there some way you could meet up with him somewhere neutral for lunch or a coffee, and just show him how much you appreciate having his company, and want to be able to see your grandchild, too? Please keep posting so we know how you're getting on. Good luck