My little sister is promiscuous?

Okay. She's 14. My father left and my mother just remarried. She goes out all night. Drinks and parties and has then comes back puts on her pajamas in gets in bed. I caught her(on accident) sneaking in when I was watching a movie. She begged me not to tell. She has been having with like 3 different guys and one I know. I'm 16. I have with my girlfriend so it would be double standard. What do I do?