She has had a serious cocaine and alcohol addiction for 10 years which she has been having therapy for. She has done well with this and we think she has stopped using cocaine. She started a new relationship a year ago. At the time they met, she told us he was a recovering heroin addict, but apparently clean now (on methodone). He seemed nice, said all the right things, we hoped he'd support her. They moved into a dingy flat, he doesn't do anything all day and frequently gets drunk (turns out he is an alcoholic), tells my sister he's going to change but doesn't do any thing about it - you get the picture.
Anyway, after another argument i helped her to move back with our (very supportive) parents last week. Now he wants them to get back together again. Says he's going to change. She knows deep down he is manipulative and a liar but loves the idea of how things could be. I have said all i can to try to stop her going back. Don't know what to do from here.