My sister is 19, her b/f is 25, been together nearly 2 yrs. they have a 1 yr. old baby. He's a cheater....

He's a cheater, lier, drug/alcohol abuser, ist, jobless, has genital and didnt tell her til after the deed was done, vehement, dysfunctional, ex-con, that has made it abundantly clear that he cares not for her future, their sons future, or much less his own. I can say all of this from 17 years of knowing and growing up with this guy, watching him, he's even been at some of my parties. I know what kind of scuzzball he is. I told my sister that 2 years ago and now she is finding it out the hard way. She knows he has cheated on her and with whom. The same chick he cheated with also came to the hospital via his invite when my sister gave birth to their son. She knew already and yet didn't go nuts on her. She cries all the time about him. I know how naive she is and its very bothersome. Heres my dilemma, my Mother, my Wife, my brother, my in-laws, even my 6 year old daughter does not like him and want me to step in, Im the one they call on to handle certain matters that they can't. The thing is we are not children anymore, not teenagers, we all have kids of our own now and age has complicated some things. I can't just beat him up and the problem is resolved. Anything I do affects the future hers and mine. However she doesnt want me to interfere, much less be in the same vicinity as her b/f. I respect her wishes, or at least I want to and I have thus far. We are the closest in our family and I love her dearly so I try not to "big brother in" anymore but, this crappy relationship she's in has gone on long enough without hope. My family and I all feel she doesnt leave cause of her son and she's scared of the outcome if she did. She has had a ton of abuse already from my father and other gender related pricks, so I've made a career out of protecting her and making her "problems" go away. So I need to know, should I perform another magic trick for her and poof he's gone and she can just pick up where she left off. Her son and her can have some chance at a normal existence or do I remain neutral and allow her to live her life as she requested. It hurts and is driving me insane to watch this happen knowing not only is he at fault but, so is she, especially after the heartache and sorrow he's already put her through. So I leave it in your capable hands, your feedback will be the deciding factor in what I will do next. I thank you kindly for your time and opinion. I would also like to note that what has been stated above is factual information, not made up, over protective, misguided, older brother talk.