My friend moved here to go to University and left her cat with her brother. It's been about a year, and her brother called, screaming that if she didn't come home and get her cat, he was putting it down.
If she can not take care of the cat then I know he could hand it in to a shelter and your sister will lose her rights to her. Since the cat has been living with her brother then he might be able to if she does not come get it. She should convince someone else to take the cat if she can't.