I've posted this b4,but need more answers my sister needs help from her abusive boyfriend?

My sister has been with this man or should I say boy for 2yrs or so,they have a beautiful little girl,her boyfriend already has one kid from a previous relationship of 5yrs before he met my sister,now to make this short his baby's mother literally lives right down the street from them,as with every... show more My sister has been with this man or should I say boy for 2yrs or so,they have a beautiful little girl,her boyfriend already has one kid from a previous relationship of 5yrs before he met my sister,now to make this short his baby's mother literally lives right down the street from them,as with every relationship I understand they go through arguments and problems,but my sister bf just takes it to another level,he has busted her lip pull a knife out on her,cut the palm of her hand while she was blocking and he then have the nerve to say sorry,he has locked her up in a basement tied up and all while she was pregnant and just this past weekend on Friday I witnessed his temper and his abuse myself,after my sister finding a phone number that he had 3 weeks ago,she wanted me to call the girl's number from my cell,while my sister's bf was sleeping in the other room.After finding out that he had met this girl at her place of work last Monday and that they hung out for the night,my sister of course was furious, her bf who we thought was asleep over heard what we had found out, automatically they started to argue,I then went to the other room where my niece was cause she was crying to calm her,then I hear something slamming and I came out to see him in front of my sister face looking like he wanted to hit her so I said your not gonna do this in front of me and get away with it,so he went into the room and took out a long knife and charge at me,at this point I'm trying not to show any fear but hoping my sister would say or do something,so she finally yelled out for him to stop,then he got in her face with the knife while carrying her daughter,What can I do as a sister?When my sister doesn't see how dangerous he really is,it's like she does but doesn't take him serious,I have offer for her and my niece to move in with me and she would agree but never falls through with it,I don't wanna call CPS cause they can complicate things by taking her child away and involve unnecessary bull-sh*t.How can I convince her and help her make a solid plan on leaving him, she already has told that she isn't happy at all with him but does not know where to start to move on away from him she is only 22 and he's like almost 30 he's too damn grown to be acting this way.Please help with sincere advices only
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