Advice anyone?

Ok ladies I need some help or advice here. I feel like a bad sister and I'm not sure what to do. So here's the situation my younger sister (17) has been talking to a boy she goes to school with and he has a girlfriend, no big deal right their just friends, but the thing is they hang out all the time like he takes her on dates, and FaceTime every single night all night long, I told her that it's kinda not fair to her or his girlfriend that he's doing that and she says "it's ok his girlfriend knows we hangout " I tried explaining how it's wrong and asked how she would feel if she had a boyfriend and he did that to her. Now it's to point where if anyone says anything to her about his girlfriend she has a fit an gets mad an yells an bitches at you for it. She sees nothing wrong with it and he has no intention of breaking up with his girlfriend. I just don't want her being used or getting sexually active with this guy. Am I wrong for trying to get her to see its a bad situation or is this something that's normal for teens now?