My girlfriend's little sister has a thing for me and she is being REALLY forward. Need some advice.?

I meet Jayne when I went to do a internship in Minneapolis for accounting. She was in the city over the summer for an internship at the same firm, we both go to the Univ. of Wisconsin. We have been dating exclusively ever since we met. I'm head over heels crazy about her, and things are great between us. I've met her family, they seem cool enough, but ever since her little sister has been calling, emailing, texting and whenever we are in one another's company she becomes really forward in a ual way. She's a good kid, but she's just started college (19 yr old) and needs to grow up a bit. I've been as blunt as possible, but she doesn't seem to listen at all. I'm even more afraid that as she is going to school she'll get mixed up with some piece of trash guy and get hurt. I don't want to tell Jayne that her sister is a slut, but I'm worryed. really don't know what to do. The whole thing is really got me on edge. Someone give me some decent advice. Thanks much.