What to tell my sister??!!!

So my step sister and I are like really close after her mom and my dad damn near tried to make us kill each other. Long story short...

Her man is cheating on her. She knows but I don’t think she’s ready to move on. She keeps letting him in. She’s smoking ciggs a lot more when she was doing good tryna quit. He is a DOG! She got pics of other girls texts and I seen him with another girl in his car. I grew up with him and his family but she met him through my brother. I gave her a heads up how they all were. She didn’t care. It breaks my heart cuz she is a good woman. His mom belittles her and I had to tell her to mind her business because my step sis is a lot of things but she isn’t a bad mother. She has 2 boys. I just wanna know how I should tell her what she’s worth.???! I really do love her and don’t want her getting hurt. He has been abusive to her in the past but she just won’t listen. Love sucks.