Hey jill,
if I were in your shoes I would most definitely sit his sister down and talk to her about it in private....Don't lash out on her or make her feel like she's a little girl who doesn't know any better...Treat her like an adult and maturely ask her why she feels like she has to be with so many people...Ask her if she wants to end up being pregnant by a guy who she was with for one night and never hear from him again...Point out all the risks that come with multiple sex partners(intercourse or not)....Stds, aids, pregnancy...She's still young, she can still direct her life in the way she wants...And i'm sure she doesn't wanna be stuck at home with a crying baby while her friends are out at the movies hooking up with the *cutest boys*....She's 15, so what, she's a soph in highschool? Damn that was only last year for me....And yep I was out at the movies, hooking up, parties, drinking, having the best time ever....I'm sure she doesn't wanna give that up...I know I have absolutely no problem *growing up so soon* because being a mom has always been such a dream for me, maybe not this soon, but i'm thankful I can even have kids....I should become a motivational speaker lol I have so much to say about this stuff...But point is, however you approach this, make sure you do it in an adult manner..Or she'll shut you out...Be calm and chill about it, but alert....And i'm sure she'll respond positively....Best of luck girl...Let me know how it goes!
Much love,
nessa & peanut