It sounds like an under-exercised cat. Your sister has to help the cat expend the energy by either walking the cat or playing with it (hunting play not some stupid little petting game). She needs to chase a feather on a rod for 10-15 minutes twice a day. She sounds like an indoor cat. I feel really bad for it since no one is taking care of her. She also needs proper scratching posts in proper areas. Poor cat. No offense but you should blame your sister for not providing the right care rather than blaming the cat.