My sisters boyfriend?

i love my sister to peices and we are very close, she came out of a 7 yr relationship last yr and then met a man (39, she's 36) 11 months ago and they have been seeing each other since. This man has two children from 2 different partners so hence take up his weekend and my sister is left looking after them with him. it has just been found out that he smokes weed, lies, conned my sister out of money and also opened a friends letter which went to his house by mistake and stole the tesco vouchers. He went on a lads holiday (my husband including) and fiddled some of the lads out of money but would never confess. I have now said to my sister that he is no longer welcome at my home for fear that he may steal something, my sister is ok with this and understands, what i don't understand is why is she with him. she says she isn't that into him any more and that he is a sbag and then phones me up to tell me she is at his house staying. she came over for dinner on mothers day, ate and then left so she could be with him. My sister is a smart person, she has a fabulous 65,000 grand a yr job, fab car, fab home which is her own and yet there is something to this man whom yes does work, but lives in a small room, drinks and smokes cigs or whatever all day and can have really dark moods. My sister who is beautiful anyway always dress smartly, is always looking fabulous wherever we are and yet has started to look terrible lately and i'm worried. NO she is not on drugs. i have talked to her but am finding now that she is becoming a little cagey due to me saying he wasn't welcome at our home. should i talk to her again? god i don't know what to do, i have never fallen out with my sister but am having to deal with these anger pangs i'm getting because this is so out of character for her.