A bit of background so as not to drip feed.
My sister is 48, divorced with two kids 19 and 21 who still live at home (but are hardly ever there). She lives in the south of England and I live up north so we don't see each other than often - a few times a year but we phone, chat on Facebook that kind of thing. I have two other sisters, one that lives close to her and another the other side of the country and my mum lives up north with me.
My sister has been involved with a man for the last seven years. They have never lived together. Okay, he is a nice enough bloke and i've gotten on well with him the handful of times we've met, but since day one i've never thought he was right for my sister and she has fully admitted she also had doubts about him. When they met he had just divorced and his head was "up his arse". She has however fallen madly in love with him and is completely besotted. The "relationship" has always been on his terms. She constantly waits by the phone to see what he's doing before making plans herself. More often than not he sees his mates all weekend and will happily tell her he would rather go for a drink down the pub with his mates than spend an evening with her. They never go out for meals, cinema, holidays or any of the conventional stuff you do when you are in a relationship. In fact i'm not sure what to do do really! 
She has been unhappy with the arrangement from the offing. Always complaining to me about how he never suggests doing anything, how it's always her putting everything into the relationship and she never gets anything out of it. They have lots of mutual friends so always seem to end up at the same parties and so end up going home together.
She often tells me things like if she sees him in the pub he won't speak to her. Or sometimes she'll ring him up and ask him what he's doing at the weekend and he'll say he "doesn't know yet" but "thinks he might go out with friends for a drink".
He has also told her mutual friends that he isn't in love with her but he does have feelings for her as a friend and doesn't want to her find anybody else.
For the last six months she has been trying to end the relationship. She has talked to him about how she feels. That's she's not getting anything out of it, that she wants the relationship to move forward but doesn't feel like he does etc. etc. He has told he that he loves her and that he doesn't want anyone else to have her but in the same breath has said that he "doesn't know what he wants", and...."he wants to meet new people".
Then he usually fucks off for a few days and doesn't ring her and she's left feeling like shit.
She came to visit me six months ago and I tried to talk to her but we were going round in circles. I told her what I thought about the situation and that she deserved more etc and that she needs to stop contacting him and keep herself busy. She spent the whole time on the verge of tears.
Forward wind six months and she's still in the same position.
She invited him for Christmas dinner but he never told her he was coming until the day before (arsehole). I assume this was because he never got any better offers.
She has been ringing my mum constantly in tears for the last few months and i'm really worried about her. Her kids are worried about her and keep buying her things to try and cheer her up (bless them).
I've got to a point though where I don't know what to say to her anymore. She keeps ringing saying she wants someone to talk to but it's the same old, same old. She is coming up to see me in a couple of weeks for a few days and I just wanted to know whether you ladies have any pearls of wisdom?
I'd love nothing more than my sister to be happy. She is a lovely person, usually very strong and I want to see her settle down with a nice bloke. I feel she is wasting her life (seven years already) with this bloke and needs to move on. Do I just but out and lend her my ear every now and then? I'm worried she will have some kind of breakdown or do something stupid.