Is it normal for my sister to be like this?

So, she's not exactly my little sister, were just VERY close and have known Eachother since we were babies. I've always been the older sister to her and have been sort of a role model, and I was not very well behaved, and she's going in the same path I have. So she's been going out with my ex boyfriend (I don't really care about that part) but he is ALOT older than her and she's like 14. I've talked to her about breaking up with him, like I did, but she thinks I'm jealous and wants him back, even though I dont. He treated me like crap, told people all my secrets and things like that AND cheated on my with his ex. And he did that with his ex. He's a piece of crap. Anyway, she thinks that he loves her, and he cares about her. It's ridiculous. So, a friend of ours told me that my sister has been telling her all the things they've been doing behind my back, she lied straight to my face. 1) why would she do all this? And 2) how do I make her stop?? (Asking & Begging HAS NOT worked.) I would really not like to tell her parents. Her mom is more protective than me. She would go on lockdown.