I would have a better idea of what to do if I knew if you both went to the same college or not. But if so, write a note TO: some other chick FROM: Her in her handwriting.
On the note, you should include your sister's name. Perhaps like it was folded, and she wrote her name on the outside, and then passed it to the other girl.
The note should read Hey, (other chick) I slept with this guy and it turns out he has an STD. What should I do?
Now casually discard this note in the hallway in front of one of her classes.
Sit back and watch the action. Awww yeah!