AITA for not wanting my sister to go to an event?

My sister wants to go to a fancy dinner and we had planned for 2-4 to eat there and we were planning for it to be a party. When she called her parents, the parents said they were already there and she is very mad because she has to go to a party with her friends. I'm just not planning the party with her but it seems crazy. How do I make the best of the situation?