Sea, that's tough. I have struggled with all of my siblings, to the point where I have very limited contact now with three of them and none at all with the other two. By limited contact I mean texts and the odd get together with one, a Christmas email exchange with another and a Christmas and Birthday phone call with the third.
My personal feeling is that families can be difficult even with any major health problems or dysfunction. Add narcissism, favouritism, abuse, etc etc and it just becomes a whole shit storm to deal with, not least, I think, because each interaction brings up a thousand things that happened before.
I have also struggled over the years with watching abusive, dysfunctional people who've done terrible things to me receive large sums of cash that they haven't had to work for and live very comfortable financial lives, whilst I have struggled on a low income, scrimping and scraping and going without. I've found myself to be very judgemental at times, to people I know who blow a lot of cash on unnecessaries - booze, tobacco, scratch cards and so on - and then claim I am 'loaded' because I have savings. My savings are minimal and what I have is in there because I go without things like booze and fags and we don't have things like Sky or smartphones.
So - my way of coping - as it is with everything now - is to try and focus on me and channel what I feel into something else - write a poem, clean the oven, go for a long walk, sit and watch TV and eat a bar of chocolate. I have stopped trying not to feel the way I do at times. I'm trying to work towards recognising it but using it in a more productive way. I'm not finding it easy but I'm not sure, what, if anything, can be done. Feelings are there and I've never found I can dismiss them quickly or easily so I'm trying to work with that at the moment. I am glad you're not going to move in with her, though, that sounds like a disaster waiting to happen.
The only other thing I can think of is with things like the lunch you mention (I've experienced similar in the past). If you need/want to meet up, make it somewhere half way between the two so it isn't down to one of you to do all the driving. Alternate paying the bill or split it 50/50 each time. If she won't agree to that then there's nothing wrong with you not meeting up and just conversing by phone instead (which is easier for you to control as you can just get off the phone if your head starts melting).
I do feel for you, it's a shitty situation to be in xx