When I was your age, I left my home town and my little sister stayed there. I would talk to my sister occassionally and she finally told me that she was dating this older guy. Come to find out it was a guy I went to HS with. I told her that he was a complete a-hole back in HS and (if you've stayed in my little home town, people don't change). I gave her all the sisterly advice trying to be tactful and not try to be the parent. (In my sister's situation she was of legal age) In the end she learned her lesson and she came out of the situation better off. I just told her some things to be careful of and was careful of what I said. In your sister's situation she's too young. It's illegal in my state for them to be having sex. In a couple years it won't make a difference, so keep that in mind because she's thinking, well it's only a couple years so what's the big deal.

Hope everything works out for you