Split into 2 posts as it is too long 
Hi all and thank you a million times over for your posts.
And to those who shared stories of their sisters.
I want to be as honest as I can and if that makes you reevaluate the picture so be it.
Full Disclosure (1) to some eagle-eyed posters
Yes, since my life sort of fell apart I have been living back at home with my parents. More of this below.
This may lead to me being (i) more sensitive to her behaviour or (ii) culpable for it and I would like to disentangle these two facets.
I know in the below post I am going to seem very overinvolved but - well - maybe I am actually.
It is useful to me to pour it all out and see what is what, with me too.
I went away to think about your posts thinking a picture would emerge more clearly but it just gets more complicated the more I allow myself to go into it.
I am so used to how she acts I am having real trouble disentangling what is normal from what is abnormal.
I have really been thinking hard about why my sister is like this to me (and my parents, to be honest) and trying to see myself from her perspective.
To those who mentioned it, yes she is very charming to others, to everyone in fact to their faces, and has a wide circle of friends.
She is only deeply unpleasant to me and my parents.
She privately has a superior attitude to the 'hoi polloi'.
Behind closed doors she will get a bee in her bonnet about something we have done or said, corner one, two or three of us, rant at us for about an hour, sometimes more, and slamming the door and some tears will be involved at some point. If she cannot get the result she wants she will leave the house and slam the door and drive off, often coming back about 20 minutes later. Of course we are all terrified that she is angry enough to have an accident and that it is our fault she has left, until she returns.
She is in her late 30s and still does this, it will happen a few times on every visit. I don't think it has ever not happened.
She will start picking on one of us and have us cornered.
There is a different strategy for each one of us.
For my dad, whose strategy is to keep quiet and say the minimum till released, she will just shout at him and hector him with rhetorical questions, badgering him into yes or no answers, then she may have a cry, get herself a glass of water, and eventually my dad will be released pleading needing the loo or suchlike, and disappear to another part of the house.
Topics include: how my dad manages his money, his health, his retirement plans or his own inheritance (from his parents).
She will get my mum when she is watching tv and will sit next to her and start accusing. My mum will engage initially until she realise she is being dragged into a fight she cannot win and then shut down and look sullenly at the TV. My sister will mute the TV and keep on at her. She will only let my mum leave if she agrees with her, which she will eventually do to end the rant. If not resolved to her satisfaction and my mum tries to leave she will just follow her, shouting that she does not care about her and often crying at that point.
On the phone to my mum she will almost always end up shouting at her - this happens on 4 out of 5 Skype sessions with the baby.
She will list some problems the baby has had that day. If my mum doesn't address them she is accused of not caring. If she does, she is accused of telling her what to do and unflatteringly compared to the underinvolved mum of her partner.
For my mum's in-person topics, she will pick on my mum's extended family saying quite rude and denigrating things about them to goad her into defending them, her management of her health, her failure to live up to my sister's expectations (not travelling 200 miles a week to provide free childcare for her as well as hold down a job and ill health), the way my mum has phrased things and the intent behind them, the tone of voice my mum has used.
For me, on phone:
Suddenly in the middle of the conversation she will do like an 'aha', and I know I have failed whatever test she had quietly set me - it will be followed by something like 'I knew you wouldn't ask me [insert a particular question] about that. You've just switched off and started talking about yourself again. [sad chuckle] never mind I'm used to it. Go on' or after I have finished telling her something a pause and then 'is that it. You're not going to ask me about my day then. Fine'. Sometimes I get, randomly in the middle of the conversation, a long sigh. I say 'what?' She says '...nothing' in an eyerolling voice.
In person, I cannot be allowed to have an opinion or point of view - she will almost reflexively start to pick holes in it, recite to me all the arguments for and against to demonstrate how primitive my thoughts are, very very often she will tut and say 'I see people like you every day in my work - organic flowery woolly people and you are dangerously ignorant' - Reflecting on this this is another thing that has really affected my self-image - she is very scientific [so am I if you look at my qualifications!] but have no confidence in this anymore - even if I present journals and studies to her she will totally dismiss me out of hand and I feel like some divining daily mail cancer headline believing astrologer.
I even know on some rational level, like the incident that caused me to start this thread in the first place, that her counter arguments are bullshit, and in a later conversation she'll take the exact opposite stance and I'll be left totally confused and spinning. If I call her up on it she weaves her way out of it and makes me the idiot again.
Very recent example - there's a [?lowbrow - I don't care, as I love it] TV show I love. She started me watching it - she was on the exercise bike (all these egs. are in our parents' house) and watching it on TV in the next room and told me how she was totally sucked in even though it was lowbrow and some heartwarming things about it.
A year on, she would sit next to me repeatedly telling me how shit it was and how stupid I was to be interested in it, getting herself into such a frenzy she told me to switch it over and if I wanted to continue watching it to get out of the room. I did
A year after that it was on when I visited her house and was watching it while they were discussing furnishings in their new house, while he continually muttered underhand comments about how shit it was and things about the type of people that watched it, but in a much more roundabout way than she would.
Two weeks ago, she was again sitting with my mum and I telling us how shit it was and how thick the people on it were, 'only x/y of them went to college' (why does that even matter!) and I said, actually, some of them are too young to have made that decision yet - and then she regaled me with the exact ages of them, and detailed their careers so far and planned, to win her point. I said 'I thought you hated them and they were shit - how do you know so much about them?!' She turned it around on me, saying anyone who kept up to date with the news would know, and because I was so ignorant and wasn't a normal person and didn't read the news, I was not only wrong but abnormal. No conversation is 'winnable
That is if I can even get her attention in the first place, as mentioned before.
I think actually sometimes what I say/think challenges her on some level and she has the bit between her teeth and will not let go until she has beaten me down.
If she ever calls my attention to something she wants to show me (either a news story, something her partner has sent her - cats on youtube etc, or one of her friend's babies on Facebook) and I for any reason dare decline to look/hear the story I get 'it's so frustrating. I can't tell you anything. You're so funny about things, you won't even look at a simple news story. It's so sad, I feel like I can't tell you anything [shakes head].'
Anything i show interest in is trivial, materialistic, the sighs and eyerolling I get whenever I show her something I have been doing or something I like or am interested in, is utterly consistent and doesn't deviate. That is if I can get her eye direction off her iPhone, which she makes a show of being absorbed in. Many of you asked the question WHY WOULD ANY SANE BEING GO BACK FOR MORE I have no idea. But I do. did. God this sounds stupid. But no matter what I do she makes an art of showing how trivial and uninteresting it is, that is if she can be roused from her phone and hear her name, and subsequently hear the question without 'completely missing it'. why do i keep going back for her approval like a whipped puppy God I have no idea. I have no idea. It is a fight I'm never going to win is it? Arrrrrgh this is sticky stuff. Better out than in I suppose.
Thank you for what you said about EA. Joining the dots.
You know I bought the Lundy Bancroft and Pat Craven books looking for clues to her partner, but saw them in her and it freaked me out.
Maybe it can explain why I'm so irrationally pathetic around her.
I particularly shared the imagery of being the colostomy bag - the rudeness, shouting and denigration my parents and I get is something I have never witnessed her do with anyone else.
She is very superior in what she reveals to me about her attitude to others. She frequently says 'I'm not like other women' in an almost weirdly misogynistic way - the most recent example is that she is planning her wedding, and was proudly telling me how most other women 'have had wedding scrapbooks since they were 2 but I'm not like that'. Implication - she has an innate and natural tendency to focus on less frivolous things. We went to a wedding show and she was proudly telling me that of the 100 tiaras in front of her, she couldn't tell the difference - they all looked the same to her. She was also denigrating the other women in her baby's playgroup for having personalised numberplates. 'It's funny - I've just never wanted one'. This is just from the past month or so.
This is also aimed at me - she knows I'm very into aesthetic detail and the eye-rolls and dismissiveness at anything I'm interested in are commensurate with that attitude.
I wondered whether living at home is why my sister has so little respect for me. But the truth is she has always treated me like this.
When I had my career, which was quite 'glamorous' I suppose to an outsider and hard to get into, she took pleasure in having a '' for a sister and liked her friends' reactions. Then my eccentricities were exotic and of use. But privately she still acted the same towards me and my parents.
This has an echo with an anecdote she repeatedly tells me - when I was tiny she and all her friends would just get me to repeat words because they found it so cute when I said them. I still sort of feel in that role. Like that's the service I can provide for her if I can get my life to fit what she wants it to be. I'm just typing here, probably making little sense, just feeling better out than it.
On car drives for as long as I can remember she would start belligerently ranting at us all for some topic or another and would not stop for the whole car journey.
She still does this if in the car with either me or my parents or both.
About three times in our lives (when she's driving with just me and her) she has stopped on the side of the road and told me to get out of her car, and I have walked or taken the nearest train or bus home
Equally when she has failed to get the desired result from an argument and my parents have been driving, she would demand they stop and let her walk/train home or just jump out at a traffic light and storm off. So of course we would all be petrified with worry.
Obviously I still have a room in my parents' house (the smallest).
But she has demanded a bedroom for herself, with all her belongings in the closets, shelves and drawers. She still has her old house, which she rents out as a buy-to-let now, and is in the process of buying her new one.
In addition, the largest bedroom (my parents sleep in separate rooms for health reasons) which used to be my dad's, she has taken over for the baby, and his cot is permanently assembled in it, and she sleeps in the large bed when she is here with him.
She visits with the baby about once every two months.
If we reorganise anything or leave any evidence whatsoever of having used the room (including moving the baby's creams and changing things from one surface to another), she goes ballistic when she gets here. If we have left anything of our own in there she starts throwing the things out into the corridor or roughly throwing them into the room of the offender.
If we leave the curtains open we are shouted at because it is too hot for him, if we leave them shut we are shouted at because it is too cold.
My dad has moved into the original (small) room my sister had claimed for herself, and has a tiny desk to put his books on. He is too scared to move back into his room in case she goes ballistic at us.