You totally did the right thing for your family. You should have no doubt about that.
Did your Mom have any idea this was building up or was she blind sided?
I wouldn't try to call her anymore. Let her reflect on her own poor behavior and see if she can see where she can change her situation. If she can't come up with a solution then SHE will be shunning your son also, that will be her doing not yours. I'm thinking she may be feeling alittle trapped in her own web right now.
You could maybe send a card telling her how much you love her and respect their relationship and do not want to deprive either one of them but................Your family comes first.