JMO here, but I think in that situation, I would suggest doing the same thing I did with my family and my two girls. My older sister especially, claimed she could not stand my oldest, right from birth, but wanted to be included in everything for my youngest, including being in the OR when she was delivered by C-section. I flat out told her that she was to either treat them equally, or not be included in ANYTHING in their lives. She herself had three kids, and DID play favorites with them, and now two out of three seldom even talk to her anymore. (Her kids are older than mine, but my oldest and her youngest are close to the same age, and I think she felt her youngest didn't get as much attention from other family members as it had to be shared between the two "babies" at that time.) She did end up treating them (my girls) equally and fairly, and now she gets to reap the benefits- my oldest will be 20 tomorrow, and doesn't care to be involved in family stuff anymore, while my youngest, who will be 18 in July, tries to stay very involved in family stuff, so my older sister gets to spend time with her now without having to be concerned with my older daughter.