Narcissistic sister

[Somevampsarehot]

I'm not entirely sure what I'm looking for here, but I really need to get this off my chest.
I have a twin sister who I believe has a type B personality disorder (a couple of them in fact). I haven't said anything to her about it because she gets very very angry and defensive if she feels like she's being attacked. She's currently seeing a therapist because her gp believes she may have bipolar, but in my experience she doesn't have extreme highs or lows. She's running with the bipolar diagnosis (although it's not formal yet as the therapist needs to see her experiencing the high and low before official diagnosis).
I found out today that her therapist 'apparently' said to her that I'm obviously someone that believes that girls who wear short skirts and tight tops deserve to get raped. God knows what she said to her to get that response. I absolutely, 100% do not believe that at all. I'm quite horrified that anyone would even think that I thought that actually. I'm a proud feminist and truly believe that rape is always the rapists fault, no matter what the victim wore or how much they'd been drinking etc.
This is just the tip of the iceberg with my sister. She regularly bad mouths me to people in our home town (I moved away 7 years ago, but go and visit frequently) and several people that I used to consider friends quite obviously don't like me anymore. She's been like this our whole lives. She would frequently become friends with my friends and then turn them against me. She dated all of my ex boyfriends when we were teenagers (I say all; there were only 3!). She used to steal from me, our mum, even her friends, and she still steals from me and my sister. She tells me negative things that other people have said about me (e.g. her boyfriend thinks I'm a snob, her best friend said that I'm really judgemental and unsupportive, one of our mums friends thought I was too stuck up because of the way I walked etc). I've had real issues with my self esteem pretty much my whole life because my sister doesn't seem to like me very much (and according to her neither does anyone else).
The thing that irritates me the most is that I do think that I'm actually quite a nice person. However, none of my old friends like me anymore, and they don't have any time for me or my kids. Ironically, my sister is so nasty about these people and their children to me. I asked her to stop insulting these children to me (she hates all of her friends kids and regularly calls them really awful names to me, but then obviously gushes all over social media about them) and she absolutely flipped at me. I got told that I'm taking advantage of her poor mental health, that I'm unsupportive and judgemental and that I'm an awful person for speaking to her in such a way. To clarify, all I said was that it made me uncomfortable when she spoke of 2 and 3 year olds like that, please could she stop. She ended up disowning me and telling me that I couldn't be a part of my nephews life anymore. She re-owned me the next day, and the only reason I didn't tell her to eff off is because of my nephew.
I just don't know what to do. How am I supposed to act around her? Or react to the awful things she says to me? Anything I do say seems to set off massive arguments and then I get bad mouthed to all and sundry. I worry about my nephew too, and the affect she'll have on him as he gets older. I guess a part of me is jealous that someone like her can be so popular and loved even though she's so horrible, and I don't really have anyone apart from our family (who all know what she's like). Any advice on how to deal with someone like this would be appreciated.

My (totally layperson) instinct is that she ascribes to you all of the negative qualities which she sees in herself.

IE She thinks that your mum's friend thinks she is stuck up. She doesn't like this label/idea of herself, so instead she tells you "Mum's friend says you're stuck up cos you walk funny!"

I don't have a twin, but my sister and I had roles in the family as well, I was the clever one and she was the pretty one. Our dad flat out told us this fairly frequently. As a result I developed severe anxiety about achieving academically and actually dropped out of 6th form as a form of rejection of this label. I still have immense problems admitting that I don't know the answer to something, or asking someone to clarify what they've said. My sister has had horrible problems around her appearance, defining her self-worth in relation to how "hot" she is/feels, with any perceived imperfection sending her into a deep depression. She still seeks validation based on her appearance, and I still seek it based on my cleverness. It's a lifelong sentence and it takes real self-awareness and hard work to break.

(I should clarify though that thankfully my sister and I are very close and our shared rejection of the labels our birth family gave us has made us a real team.)

I don't have that twin bond, stepmother. Not sure I ever had it tbh. I don't know if I'm in denial but I don't love her, have no compassion toward her. I don't wish her ill I'm just blank about her.

Pp saying my twin didn't want a twin - this absolutely fits my twin. It's primal with her: it's as if she truly can't bear the idea I exist. It is anathema to her. I'm surprised she didn't kill me in the womb (or perhaps she 'thinks' i tried to kill her in the womb? I was the stronger, bigger baby, out first ; she had a very difficult birth).

Anyway. . I really cba with all this shit. I've done it to death in therapy and I'm sick of it. Enough already.

Ok, well thank you for your input @springy but I'll probably continue to discuss it as I haven't done it to death in therapy and I'm appreciating all the advice I'm getting.
We're fraternal twins, but I definitely don't feel a strong bond to her because of that. I'm closer to our younger sister actually, and people always assume we're the twins because we're a lot more similar than my twin and me.
I think she loves the attention being a twin gets. There weren't many of us growing up (no other twins in school etc) and I think it made her feel special. She definitely doesn't like me getting any of the attention though! She often has medical or emotional 'emergencies' during special moments of my life (when I was pregnant, at my wedding, when I got engaged etc). She got very vicious when I went to uni and became a fair bit more confident (started messaging my friends and boyfriend, all of whom found it incredibly bizarre). I can't tell her if I'm upset with anyone because she either messages them or plasters it all over social media under the pretext that she's trying to help. It's mortifying.
When I was pregnant she emailed some mutual friends (all male, all of whom I'd casually dated previously) mocking how my body had changed. She had Googled images of similar things and sent them to them so they'd know specifically how dark my nipples had gone, for example. She left the messages open on my laptop and I'd seen them when I went to turn it off. I was only 21 and I was absolutely devastated. She's always done things like this. She used to take photos of me and my sister on the toilet or in the bath and send them to people when we were teenagers. I'm getting very angry and upset writing all of this down. I'd forgotten just how awful it was living with her. She was just so vicious and nasty at times.
Sorry, I got lost on a tangent there. Thank you everyone for your comments. I do feel like I'm making the right decision by reducing contact with her.

Weird and horrible behaviour - as if she wanted to annihilate you by stripping you of your privacy and dignity. Vile.

Wow, that's pretty shocking stuff. She really really doesn't like either of you, does she!

You know, I think LC/NC is really the only way forward here.

Parents? What do they think?

Oh yes, sorry! My mum and dad broke up just before our 3rd birthday, and up until we were teens our dad wasnt really involved in our lives too much. We stayed with him maybe 3 or 4 nights a year? He tends to go back and forth on his opinion of what she does. At the moment he's in a 'it's his fault for leaving, she's sick and I need to be kind' mentality. But he's said before that she's a manipulative, jealous person and that he was ashamed of her. He hasn't said that to her, that was said to me shortly after I went to university. They have a very up and down relationship; she either idolises him or hates him, there's no in between. My mum feels an enormous amount of guilt. Both because she didn't manage to stop my sister acting like a monster and also because it never occurred to her that she could be mentally ill. Since I left my poor mum has become a shadow of herself. My twin didn't move out until last year so it was just her, my mum and step-dad after my younger sister moved in with me 4 and a half years ago. My twin has been incredibly abusive towards my mum at times and has slowly chipped away at her self confidence too. It's awful to see; my mum raised us on her own for 15 years and was my superhero. She was independent and strong and didn't take any crap from anybody. She often avoids giving an opinion on anything my sister says because if it's not right my sister will send her essay after essay about what an awful mother she is and how unsupportive she is (very similar to how she responds to me). In the last year or so I've gotten a bit braver and have started calling my twin out on her bullshit. I have always rebelled against her slightly and I think that's why she's so vicious to me specifically. My younger sister just nods along to whatever she says and therefore my twin doesn't feel so judged and hasn't been as vicious to her over the years.

Oh dear, that really does sound terrible. Your poor family. :(

We're on the same side here op, let's not forget that.

@springy perhaps I misunderstood your last post? I read it as you were bored of me talking about my experiences and basically telling me to shut up. But maybe you meant you were leaving the discussion as you've dealt with your issues enough already? If that's how you meant it then I apologise for my shirty comment.