Dude, if your sister was under 13, it would have been creepy, but she's not. She knows what she's doing (and until she hits 16, it's go after the first guy that says "I love you", seriously listen to Taylor Swift- 15). If something does happen, she will learn her lesson. It's great that you look out for your sister, but there are boundaries to that too.
Chill out, when she brings another dude in, just chill with him and get to know him. It sounds like you're trying to take the rebel-ness out of your sister and conform her to preppyness.
