It is up to your son and his wife to clarify this situation with your daughter - not up to you. I'd be telling my son to phone or text his sister and making this very clear. It's cowardly of him to expect you to enforce his new "rules".
Having said that, unfortunately we can't always make our children remain as friends when they grow up. My two never speak to each other except at family events, which I find heartbreaking, but you can't make people like each other . We can only live with what we've got and love them all . Good luck to you dealing with this awful situation.
SNP and Greens end coalition deal
Scottish political mess. Is Devolution working?