Concerned about my promiscuous little sister? Christians ONLY!?

She's 14. She sleeps with anyboy she thinks is cute. She's in the "popular" crowd at school and that is amazing to her bccause she is a freshman. She mocks other kids that she doesn't like. She dressed like a slut to attract attention from other guys. She parties till all hours of the night and comes home super late but she puts on a "good girl" act for my parents. I'm torn wether or not to tell them. She is into paganism and stopped believing in God at 11. How can I help her out of sin and into a relationship with Christ?