Idea:
Work really hard at school. Become smart as hell. When you get a job, you'll end up in a well paid job and earn lots of money.
Meanwhile, while you're rich, your sister becomes a drug addict. Seeing that you have loads of money, she will eventually ask you to borrow her some. Give her $2000, or so. She will spend it all on drugs, naturally. She will then think you are a caring and kind brother, and will come back to ask for more money. Simply say "No" and shut the door in front of her. She will now hate you, herself and the rest of the world. And then she will commit Self Delete.
Best payback ever :D