This may be a long one, but it's all relevant to the current problem. Some of you might remember my Christmas post To hate my sister about my miserable Christmas staying at my mother's house in my home country, with my adult sister behaving awfully.
Unfortunately, it's still rumbling on.
In one of my posts on that thread I mentioned hiding in my mother's car on the driveway to get away from my sister, because she'd been following me around screaming at me. What happened in the leadup to that was this. My mother and I had been supposed to go into town shopping in the morning. When I got up that morning my sister was in the kitchen with my mum and dad, and I could see she she was in a bad mood and about to kick off, so I absented myself and went for a shower. In the shower I could hear raised voices, and when I got out it had escalated into screaming, so I got dressed, grabbed my bag and phone and just left the house. My mum met me in town several hours later and we did go shopping, though it wasn't exactly a happy atmosphere.
My sister had gone into town on her own, and met us to get a lift back home from my mum. I was very angry with her, but I didn't want a row. As we were getting out of the car back at the house, I said to her that I thought she needed psychological help - my honest opinion, based on her behaviour. She flew into a rage, started screaming at me how dare I insult her, etc etc, and when we got into the house she went straight into the kitchen to my mum. As I'm taking off my coat my mum then came out looking furious and said 'Apologise to your sister' I asked what I was supposed to be apologising for, she said for insulting her, I said, quite honestly, that I wasn't insulting her, my opinion was that her behaviour was very far from balanced or normal and that she needed some help.
So screaming and shouting from my sister, as I move around the kitchen trying to get something to eat, along the lines of 'HOW DARE YOU', and then she proceeded to rant and rave at me saying:
What would you know, LIVING ALONE IN ANOTHER COUNTRY [??]
At least I'm not living with SOME BOY THE SAME AGE AS [sister] [again, wtf? partner is a year and a half younger than me - not much in your 30s] who's too FAT AND LAZY to come visit her family at Christmas
My mother had gone into another room to avoid 'the row' - ie my sister being abusive to me, and my dad was ignoring it in another room, being the main target of my sister's abuse. At this point I stuck my fingers in my ears, grabbed my stuff and went to sit in the car. My mother came out twice to try to get me to come inside - because my old friend was coming to pick me up so we could go out for the evening, and my mum told me she didn't want my friend to see me sitting in the car and thinking there was trouble in our family. ...right.
My partner and I love each other very much, and are planning to get married and have children. His family are lovely. He didn't visit with me for christmas because he had bought a flat recently (which we both live in) and with having to unexpectedly remodel the whole thing once chipboard had been removed to reveal crumbling walls, simply did not have enough money for flights. My family had been told this well in advance of Christmas. We'd been calling and texting regularly, also because I was worried about visiting, as my sister had been awful the year before as well. So, I called him, and I told him everything. He was understandably pissed off. I managed not to see my sister again much until I left, but I was gutted and exhausted by it all.
Since then, the following has happened:
A few days later: My sister 'apologises' via text 'I'm sorry BUT you called me a name...'
I let my mum and sister know that my partner does know what happened.
My sister says I shouldn't have told him what she said as we were 'having a domestic' - um, no, she was following me around screaming abuse in my face.
My mother says 'family things are to be kept private' and blames me for 'causing upset' by telling him what happened. I tell her he is my partner and the man I am going to marry and that we keep no secrets from each other.
My mother reiterates that 'the row' was my fault because I called my sister names - FFS...
I asked my sister to apologise to my partner, either via text or a phonecall - neither of us would have hung on to it if she'd just apologised. I suspected she wouldn't as she honestly thinks she'd 'done nothing wrong' but she eventually said she would apologise. She never did - oh well.
The current problem is that it is her 30th birthday soon and yesterday my mother rang to tell me that she would like us all to go out to dinner, and can I and my partner come over to celebrate it. I said I could fly in on a saturday and back out on a sunday, flights are cheap and that would be fine. My mother then asked if my partner could come. I said probably not. She, in a tone of great shock, asked why. I answered that sister had never apologised to him, and he's still understandably a bit cross about it.
My mother then went off the deep end and told me that I had 'better fix this' as it was all my fault and I'd caused the whole thing by 'blabbing' to him what my sister had said, called me 'big mouth', the whole deal. I got angry and rang off. Cue profuse text apologies, she loves me very much, she will get my sister to apologise etc etc. My partner and I receive perfunctory apologies by text.
Today I sent her happy mother's day message, told her about flowers being delivered. She thanked me, mentioned that my sister had told her she had apologised, so all was okay now. She said then that 'being honest what kicked all this off was [partner] not coming for Christmas', but not to worry, it's all in the past!
I'm completely exhausted with all this. Can someone please tell me how it is possible in her mind that, even with all of the hideous behaviour my sister has behaved, it is all my fault? How is any of this my fault? Have I lost the plot, mumsnet? I'm 
. I feel like I do everything right, am a nice and loving person, and my mother basically cares about me about one quarter as much as she cares about my sister. I can tell that my mum is really pissed off with me, and I just feel it's so incredibly unfair.
Another aspect to the drama is that partner and I have decided that we've stood up to them now, so we will go to visit for the dinner. I'm very worried about our staying in my mother's house as this gives sister the perfect trapped audience if she decides to go off on one, but equally I know my mother will be upset if I book into a hotel nearby, and she's already angry with me for 'causing upset'.
God I'm tired. Sorry this is so long, I'm just so confused and exhausted by it all.