Am I wrong for being mad about my sister wanting to go on her choir trip?
Why would you be mad about your sister going on a trip?
I have better things to do than watching my sister sing
Do you have to go on the trip with her? If you don't then there's no reason you should keep her from going. That's going to come across as selfish, and could drive a rift between you and your sister. Especially since this is probably important to her.
I know I can't force her to continue with this annoying trip.
It's understandable that the choir may not be something that is fun for you, however that doesn't mean you should ruin the experience for your sister. It's selfish to try to stop your sister from going on the trip.