I want my sister to break up with her boyfriend!!

So my sister been dating this guy for about almost 4 years now. She dated him when she was a sophomore and he was a freshmen. SO he is younger than her. SO as times grew summer came. They started dating in the fall, than next year summer came and he said it’s over because he liked someone else. SO right after they broke up he was already making out with another girl!How do I know? I accidentally saw him and her on a locker when I was walking out of school! So he been flirting with this girl during MY SISTER”S relationship! My sister is smart ,she has a 3.88 GPA and he is like 3.4 or w.e. She loves him a lot and with all this sh*t she has to deal with( family and school and now getting into college) she has no one except me to talk to. But I understand that she wants someone else besides me. But check this out, happened again. So once that girl was done with him he started flirting with my sister AGAIN . SO she feel for it because she really loved him. Than again he cheated on her and starting getting me mad and sh*t. I was p*sse . I keep telling her that he is just playing with you because your are so pretty. My sister is really pretty. Like she doesn’t even need to put make on!. WEll so he left that other girl and went back to my sister for the 3RD TIME!

So I said” I had it”. I love the guy, he is nice and really cute but still looks can't get you every where. I can’t keep seeing my sister be a piece off sh*t he could just throw away and pick up again. I see my sister cry some night because he left her and after that she is so ugly and down. But when she is with him she is so happy. I don’t wanna take her happiness away because she was already messed up and down without him. But I can’t handle it anymore. She is head over heels for this guy but he I don’t think he is. I feel like I see his pattern . He only wants her when he wants her, and he knows when he wants her back she will be like “ok”. And I get p*ssed at that; my sister is not some doormat you could just step on. She loves you, Yes, you might say you love her too but to me it doesn’t look like you do. I don’t wanna sound rude and selfish, like I’m taking away my sister’s happiness but I’m trying to help her get out of this. I feel like this makes her feel even lower than how she would feel if she was not with him. She loves him a lot, but she is going to college, close to home, so she could always see him, but I don’t care. I don’t want her to see him anymore once school starts. There will be better guys in college, ones you could relate too. But she says she does not wanna look anymore after him and/ or there is no other guys like him. WTF! But it’s her life. am I doing the right thing? I wanna have this talk. I wanna talk to her about this and I want to talk to him. He drinks now, kinda been like for the past 6 months and my sister is like could you stop in a nice way .He does at times but he could lie to her. And he is a good liar. What should I do?
I want my sister to break up with her boyfriend!!
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