I'm going to dump my cat on my girlfriend's sister without telling her.
Why are you going to give your cat away?
I hate the little shit and I don't want it anymore in my life.
Dumping your cat on your girlfriend's sister is an awful thing to do because pets should not be treated as objects to be given away. The cat is your responsibility, and thrusting that responsibility onto someone else isn't the right thing to do.
I really don't care what happens to him.
When you get a pet you need to take care of it for its whole life. You are all the cat knows and just to abandon it like that is a horrible way to treat it.