I'll say something else. When the niblings were little right up until they were actual, legal adults I had a pretty strict rule: Grown-up problems are for grown-ups. I didn't talk about my problems with friends or - well, I certainly wouldn't have badmouthed my sister to them, she's their mom, but I would not have insulted some other hypothetical relative to them either. My relationship with all those other people is my business, and I sincerely don't think it's appropriate to drag kids into that unless either a. they bring something up or b. it's necessary for their well-being.

And I don't think this started just after Grandma died, either, when Daughter was already in her mid-teens.

This sort of thing just feels to me like it edges close to asking your child to manage your emotions, and look - Daughter decided to take it into her own hands to "punish" LW's sister.