Hi everyone,
Thank you all so much for your posts - they’re really helpful.
We’re fraternal twins, so non-identical.
In terms of the background to what happened last night, my sister hit me hard across the mouth first after we’d argued.
She told me it was an accident, as it seemed almost if she just let her hand go rather than if it was a controlled hit.
I reacted by hitting her back, as it was painful and I wanted to get my own back.
I’m ashamed to say that, in the past, I have hit first at times where we have fought and hit each other.
When we were much younger (probably below the ages of ten), we would sometimes fight physically. I would react first and hit my sister first. Mainly, I think, because I would be frustrated at her and didn’t have the strength to walk away from the situation or the quickness to react with a verbal reply (ie. with something that I knew would hurt her, as I wanted to hurt her as much as I’d been hurt by whatever she’d said to me).
When we grew into teenagers, my sister and I would still fight, and it was mostly verbal. I can’t think of any specific times when we had a physical fight. It is very, very rare but can still happen. I really need to address this.
With the verbal fights, my sister would often start it off by picking up on something that she knew I found very hurtful - ie. she would tell me I had no friends and that no one liked me. For background, I was bullied at times in secondary school by people in my own friendship group. As a result of this and of my sister using this as ammunition, I had very low confidence in secondary school and didn’t really enjoy it at all - I now don’t have any friends from secondary school as I found it very difficult to make friends. I also found it hard at the new school I moved to on my own (mentioned in my OP) after our GCSEs. My sister can be very territorial about her friends, which is odd but also understandable. So I’ve learned not really to socialise with her friends.
I also always really admired my sister when we were growing up (she’s very pretty, she’s incredibly academic - had brilliant GCSEs and A levels, and just sailed through school - and always seemed to have a good amount of friends to be quite happy at school. I was much happier at university. I worked very, very hard to get into one of the Oxbridge universities (would rather not say which one) to firstly make my mum proud of me, I think, and also to prove to my sister that I could do it (my sister was always very academically gifted at everything, whereas I never really understood maths and science and loved the arts and languages).
When we were children and teenagers, mum would then hear us arguing or fighting (she’s a SAHM, so she always looked after us full-time as children), and she would come and try and stop the situation by screaming at us to stop and by hitting us (often with her hand and with some force, in the heat of the moment). I think she didn’t mean any harm but I was actually a bit scared of her (and can still be now).
Our mum basically brought us up alone until the age of sixteen, as our dad worked abroad, apart from a couple of years when we were young when we all lived abroad together. My dad would address the situation by pleading with us to stop bickering or fighting but wouldn’t react physically.
Now, my parents both live together in the UK and my sister visit them every couple of months (we don’t necessarily visit them together, apart from at Christmas).
My mum is getting older (a pensioner) and wants to gift my sister and I to buy a house and wants us to live together if possible. This is another issue and potential can of worms. My sister and I have really tried to show that we can live together but I’m not sure how good an idea this is (unfortunately, it wouldn’t be an option to buy two separate houses).
When my sister and I were growing up, my mum used to always compare us to others (always!!) She still does, now. She’ll say things like, ‘why can’t you be more like X’ or ‘why couldn’t you have done this like X?’ She doesn’t compare us to each other, strangely, but frequently, she’ll compare me to other people we both know (high-achieving acquaintances at school, close family members).
It’s really upset me over the years without me realising, but it’s now made me feel rejected, inadequate and really insecure (I’ve had trouble in friendships by asking friends if they like me 
). I’ve never had a boyfriend as I’m too scared of rejection and insecure about certain aspects of myself (eg my looks). Also went to a girls’ school until the age of sixteen and didn’t socialise with boys outside of school. Tragically, spent my weekends doing homework.
However, since going to university, I feel a lot happier. I’ve made very good friends, enjoyed my degree and did a degree that I really wanted to do, and now I finally have a job I love (after spending a long time working out what I want to do and applying for lots of different things). This has made it easier to deal with anything my mum and sister say that upset me. I know they love me and I love them, and we are all close, but we struggle to get on well consistently when we’re together for a long period of time.
I’ve now tried to respond to whenever my mum says this by saying something like ‘comparison is the thief of joy’ (lame and can’t think of anything more original, but I just say it whenever this comes up so she gets the message)