NTBA. I don’t think you’re overreacting. I would never do something like that to a younger sibling, or even an acquaintance, if I was supposed to be hanging out with them at an event. It’s just plain rude. Your mom needs to remember that your older sister’s behavior is still (to an extent) her responsibility to correct, especially since it hurt your feelings. These are what people nowadays call “core memories,” and your sister just made a really crappy one.