I understand where this grandmother is coming from, and I don’t think she’s being partisan, and even if she was partisan, why shouldn’t she be? It’s her son and assuming he has never abused his ex wife or children (and there seems to be no reason at all to suggest he has) then both the father and grandmother are being treated very shabbily by the mother. Why is grandma only allowed to see her grandchildren once a year? They all live in this country apparently. Visits are short because of the demands of their mother.
I have quite a lot of experience with difficult ex in laws, in my case an abusive ex son in law. My daughter and her children live abroad, although my daughter wants to come home with the children, but is unable to do so at this time. It’s a long story, and a difficult and complicated situation, which I cannot explain here, but it does give me some insight into these kinds of difficult people and situations. Of course this grandma should support her son, and she certainly should be partisan. The ex DiL is behaving badly towards her son and towards her. Sadly this is often the case after divorce. The children are used by the controlling partner, in this case, the mother, to punish and manipulate their ex partner. It’s sad, but believe me, it happens.