Your parents love the dog too. Try and consider that they're not doing this out of malice. They're probably thinking of the dog more than they are themselves. You should try and do the same. It's hard, but the dog is suffering. If the doctors say it's aggressive and she has weeks to live, then she must be in pain. A lot of animals won't show pain until it's unbearable. It's not as if they can say to you "my head hurts." They probably just don't want her to suffer too much longer.