What should I do?!?!?!?

Well (I know this isn't the answer you were looking for but...) maybe you should try to talk to your parents again. Not about him leaving, but just ask them to talk to him about it. And he's your uncle I'm sure that he loves you and your sister if all else fails talk to him yourself, that could be all he needs to realize that he has done wrong. I hope everything works out(: