Get together with your other roommates. Next time the 'art club' is held, face them together and tell them the art club can't be just at your place every time. It's not fair to the other people who live there. Each art club member needs to take a turn hosting it before it comes back to your apartment again. If they don't listen, and show up again two days later, make noise. Play the TV loudly, or games, music, or a movie. Use pots as drums. Sing the cruddiest songs you know at the top of your lungs, jerk on pots, blow whistles, etc. until they leave.
Your SIL is a jerk. If you borrow something and damage it, it's up to YOU to fix it, not the owner. Suggest you go get the flute, then tell SIL you're not letting them borrow the flute any more, since they damaged it.