Thank you for all your advice, and for those posters who have shown concern for me and my DC 
I can't answer you all individually, as much as I would like to, but I will try and cover everything.
She does have a problem. My D(half)sis grew up with her and she hasn't changed. 20+ years she has been like this. As a side note, Granny (her mum) was an alcoholic, too, to the point of hiding alcohol so we "wouldn't find out" (never mind the fog of booze reek around her).
To those who wonder why I am even asking it's because this is the first time I have faced this, with her as a house guest and finally sober myself. It's difficult to know if IABU, or just hypocritical and spoiling her fun, which is how she has made me feel by crying about it - I've stood my ground and stayed strong on the matter though, despite DH not being here for moral support!
My Step-Dad is depressed as a result of his Mums death a couple of years ago. I only found this out when they arrived and have given him the only advice I can. Stop drinking, go back to the doctor. He drinks, but handles himself far better than my Mum. He is no trouble although it is damaging his health, too and while he is in this state he can't help her.
As for getting her help, it's quite difficult. They live in Scotland and I only see her once or twice a year with very little contact in between (although not quite NC) but when I do see her this is what it's like. From a distance it is hard to do anything. I have told her she is an alcoholic like Granny was and she has either brushed it off or twisted it so it seems i'm personally attacking her. Or cry's about it.
Somehow my Half sister (who also lives in Scotland) has grown up into an amazing, sensible young woman. She hates Mum drinking too, but Mum won't accept help from her.
The only thing about going NC with her is how much pressure that would put on my Sister, half sister and Step-dad. All I want to do is tell her to go, and under no circumstances are they going to be staying with us again until she is sober.
Friends? She doesn't have any. Most of our relatives up there put up with it to an extent, but she only really sees them when it's an "event" like a birthday, Gala day, Christmas etc. They don't see that as her normality.
The strange thing is that she works 12 hr shifts, 4 on and 4 off and can quite easily handle herself without even a glass of wine when she has to be at work. She doesn't even shake because, I assume, her tolerance is so high. Bus as soon as she starts her 4 days off she comes in from the night shift, cracks open the wine and drinks until she passes out with Step-dad, who also works nights.
Thank you again, all of you, for your help. I have some serious thinking to do about this, not just for me but for the sake of the kids as well.