I feel like my little sister is ruining her life, what do I say to her or do?

She is completely out of control. She just turned 15 years old and has had sex with 10 guys. She isn’t on birth control and has unprotected sex. She does pills and smokes marijuana. She sneaks off with her 19 year old boyfriend all the time. I have no idea what to do with her. I don’t want her to think I’m trying to go against her and I don’t want her to think I’m trying to “ruin her life.” (As she says) What would you do in this situation?