Thanks all.
I think the reason I feel that diagnosing the NPD and understanding that is important is that I wouldn't feel unreasonable or awkward by not complying with what she wants and it would help me to maintain a level of distance.
It has also gone a long way to explaining the relationship my mum has with her. Whereas before I used to feel aggreived when we were supposed to be doing something and my sis would call and my mum would go running, now I would know that it's not about what's wrong with me (why does my mum prefer to spend time with sis), but about what's wrong with sis and her constant needs. I don't harbour any ideas of her changing. I just want to know if it's possible to deal with it and maintain a distance.
Atilla's comments certainly resonate in relation to my dad. I know that any involvement with him would end badly and I do intend to stay the fuck away well clear. I think because he is more likely to cause emotional damage and I wouldn't cope with that.
Re my sis it seems a little easier as I'm less involved anyway. So Cognito et al's comments about determining a level of tolerance sound like that could work. I think it would be easier to withdraw in the future as I have made it clear I have been NC because treating me the way she did was unacceptable to me. I could easily return to NC if she 'reoffended'. I also wouldn't put myself in a position again where she could find my achilies heel and strike so I wouldn't be as vulnerable.
DH is great at the support side of things and says she's no way near as bad as my dad. They are both definitely NPD but tick different boxes. But DH was a good person to laugh with about her attention-seeking behaviour and was good at putting things into perspective. As I said earlier, I also now know that it's not about me, how anyone feels about me etc, it's about how other people are responding to her NPD which really helps with my own self-esteem.
I know it's not about whether she wants to reconcile. I just mentioned that because I didn't want it to look like I was thinking about it, for her to just potentially reject any advances anyway.
I'm still undecided though. My gut is telling me to just leave things as they are as I've been happy without the contact. I don't know if I can be doing with having to be on my guard, being wise to manipulation and having to bite my tongue the whole time. I have felt a little bit excluded sometimes but I think that would still happen though because of my being NC with my dad. And they're all as mad as hatters anyway so it's not the biggest loss.
It's interesting to know that it is possible to do it in degrees as it were, that it's not necessarily a case of all or nothing and no in between. I think the challenge could be determining where the level of distance is placed and her wanting to cross that and get closer, ie return to how we were before.
Maybe I'll go to my mum's for dinner one Sunday and see how it goes. Then I can dip my toe in the water and see how deep I want to go before it gets too cold and I stop. 
Thanks for all the advice. It's been really helpful. Mumsnet has been great in helping me deal with all this. 