Little Sister starting to become promiscuous??!?

My little sister is 16, and of course she's getiing into boys. I don't care particularly about that, I'm close in age to her (18) so we talk about that stuff. But she's had a problem in the past when she broke up with her first boyfriend, she was so crushed that our Dad took her to a psychiatrist. She 'worked through it', but since then has this "I don't have to be in a relationship to be happy" mindset, which could be good, but instead she keeps hooking up with random boys. And by that I mean giving BJs, HJs, about everything else. Then she met a boy two weeks ago, lost her virginity to him, and she then finds out that he has a girlfriend. She says she doesn't need a bf, but she still constantly needs that emotional attachment. Now she's messing around with a couple more guys, and one that I think could end up being aggressive. Any advice!? :)