Hello pritcharddesign, I'm so sorry that your sister is being treated in this way. It's heartbreaking when the joy of becoming a GP, especially for the first time, is short lived.
FWIW, I think telling her son she wont be inviting them to anything anymore, because they come was the right thing to do. It will I doubt improve the situation but at least she has shown that she's not happy with how she'd being treated, and isn't prepare to take it lying down.
Apart from having and maintaining barriers, and letting her son and his wife what they are, I'm afraid there's nothing she can do.
I'll never understand why these sons tolerate their mums being treated this way, but as long as they do the situation is unlikely to change or improve anytime soon.
My advice would be for her of course to see her son when he visits, ask after her GC and his wife but leave it at that. Don't ask to see photo's and don't ask if/when she'll be seeing her GC.
Not easy by any means but being seemingly untroubled with what's going on, might bring about some positive changes.