My husband has an older brother and a younger sister. The brother and his wife do not get along with sisters boyfriend. Due to disagreements over covid/restrictions/politics, the brother and wife refuse to be around the boyfriend. Now that covid restrictions are easing, brother and wife want to host a dinner at their house - inviting grandparents, parents, us, and sister. Boyfriend is specifically not invited. So sister is also not going.
I think it's their house, they are allowed to invite/not invite who they want but shouldn't be surprised if sister doesn't go. Husband wants to also not