If You're Unhappy, Just Leave, Right?

So my sister is dating this guy named Melvin. And he's a real douche. I never liked him from the start. Yet, throughout their fighting, and him just using her as a meal ticket, she still stays with them? They've been dating since like what? January? And they argue almost every single day. She is so unhappy and I know it but she STILL stays with him. Her youngest son's father Jason dated her before and they broke up because of BOTH of them being super immature, but he makes her happy and she's like toying with his heart, breaking up with Melvin to be with Jason for like a day. She, right now is like dating him and flirting with him on the side. They have like a secret romance behind Melvin's back because she's like afraid, I suppose to break up with him.. Like REALLY? And I HAVE to deal with their bullshit every day because their bickering goes on CONSTANTLY. It's so annoying. Yet Jason STILL wants to pursue her even after I told him how much she's a SHIT girlfriend.

...So dumb. I hate teenagers.