I love my big sis, but I want to beat the hell out of her

My sister is married to a crack addict. I can't count the number of times she's started the process to divorce him and then backed out.

Over and over she says she's leaving him only to allow him back in the house. She has three kids by him (16, 13, and 6) and he steals her stuff and the kids stuff.

He goes to jail, gets clean, get out and she takes him back. After a while- could be weeks, months or once it was almost 2 years-he goes back on he crack. Stealing and God knows what else.

She's miserable, but can't seem to break the cycle.

While he was locked up this last time (she had him arrested, he went to prison for eighteen months I think it was), She met this wonderful man. He's completely different from my brother-in-law. This man is sweet, is good with the kids, has a good job, and he loves her. She has been a completely different person. She apologized to him for being a bitch (if you knew my sister, you'd know that is a HUGE deal because even when she's wrong she does NOT apologize). She was the person she used to be before all the drugs and chaos made her miserable and depressed. They were even looking for a house for all of them (my sister, her three, her boyfriend and his daughter) and then my brother-in-law gets out of prison and she takes him back and breaks up with the good guy!

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OK I started this yesterday and I have since talked to my sister... anyone got any advice? Let me continue first...

I talked to both my sister and her boyfriend. We'll call the boyfriend (or ex since she broke up with him) "C".

C says that he loves my sister and that he'll wait for her. He is crushed by this, but is being understanding and loving.

My sister loves C. She told me that. She says she does not feel anything when hugged by her husband. But she is scared. She can't "do it" on her own. What if C leaves her? She can't "do it". She said that if they had found a house and were living together, then she would be with C and not have gone back to the crack-addict. But shes scared and she says what if? What if C decides he doesn't love her etc. etc.

I made her cry. I feel bad, kinda. She loves C and he loves her, why go back to a miserable life.
I was pretty blunt, but I can't see why she'd stay where she's miserable.

I think that my sister thinks she doesn't deserve to be happy. That she is having abandonment issues and she thinks that even though my brother-in-law is a crack addict he'll never leave her.

So, do I tell C that my sister DOES love him and want to be with him, but she's terrified or do I keep my nose out and let my sister go back to a life where she is depressed and miserable ALL THE TIME?

HELP!