Will it hurt my parents feelings if as an adult I don't keep a relationship with my sister?

It's not your problem, and no you don't have to have a relationship with her. Your parents, are why she is the way she is. They should discipline her for her bad behavior. There is little you can do. Your 18 now, right?? Another few yrs, or college soon, will get you away from the brat!! Good Luck!!