Talk to her she's your sister... I find it easy to talk with my sisters, when I was a teenager, I didn't feel right talking to my mom, and I was very close with her, but my sisters could relate. However, when they didn't like a boy I did, and let me know, I stop talking to them. I know now I should have listened, but at that age, I felt I knew what was better for me. I was blinded and messed up big time.
Talk to her about the only thing guys are interested in is sex at that age. Then talk about protection and STDs. Especially like herpes and other diseases that you have to live with for the rest of her life. Tell her, you can even get herpes from oral sex. I would scare her about sex, especially how guys who really care about you, would wait. They don't want a woman who has had a lot of partners.
I had a friend in high school who's parents were very controlling and sheltered. Her little sister got pregnant at 18, the first time she had sex and the guy left her.
Just talk with her, don't go off on the guy right away, point out his flaws but don't go into too much details.
Good luck, hugs going out to you!