Hi everyone! Have name changed for this because I'm worried about outing myself. I don't post often but I really enjoy this site and post occasionally and could really do with some perspective on my relationship with my sister as it's tearing me apart inside. Sorry in advance as this may get very long but I want to explain the situation as best I can.
First of all, I must admit I feel a bit ridiculous saying this, but I feel that I'm being emotionally abused by my youngest sister and that I have been for quite a few years. For a bit of background: we are a large and close immediate family, I'm one of five kids, our parents are still together and we all see each other frequently. I'm 28 and my sister is 21, she's the youngest in the family, incredibly intelligent and confident and studying at a top university. I actually moved abroad a few years ago for work but regularly come home to the UK either for projects (freelance creative job) or purely to spend time with the family. Sometimes I'm joined by my wonderful DP for these visits. All of us siblings are married or happy with partners except youngest DSis. I absolutely adore my family and cherish time spent with them which is why the DSis thing is so hard for me.
In general I am a confident person. I have a lot of lovely friends, a great partner, I am generally well-liked and respected in my field of work although what I do is a very competitive industry and as such I deal with the usual rejection and being passed over for things etc which no one likes but you pick yourself up! In my normal life I feel like a strong, happy, accomplished person still striving to achieve more. However when I'm around her... no one can manage to make me feel such a pathetic loser. She has this unique ability to choose things to say that will cut deep to my core, zap me of confidence in seconds and leave me questioning my worth and whether I'm the black sheep of the family. (I actually feel pathetic even typing that, that someone 7 years younger than me manages to do that). Sometimes my parents or siblings will witness her explosions at me (more on that later) but she saves the really cutting remarks for when we're alone and I feel like I'm going crazy.
So, the explosions. She is openly quite a volatile character and will 'fly off the handle' quite frequently if someone annoys her but with the rest of the family she'll rage for a minute or two then simmer down and she'll be laughing again shortly afterwards. With me the rages turn into huge personal attacks and have often led to me eg leaving dessert because I can't hold back the tears. My family will sometimes tell her to apologise or whatever but I feel (and don't really know how to articulate this) as though somehow they just see us as particularly 'rivalling' siblings who just spar frequently. But the thing is that these situations are never 'arguments' in that we both get angry, say our piece etc, I am just attacked and it's impossible to say anything at all as I just get shouted down and I'm really not a loud or shouty person. Although she doesn't let the mask slip around people outside the family usually, recently she had one of her explosions at me when my DP was there (she stormed out before he or I could say anything) so he has seen what she can he like. He told me she's horrendously immature and I need to 'stand up for myself' and crush her nonsense but I just don't know how when she's a very shouty person and I'm naturally milder, she's also extremely intelligent whereas I panic and find it really difficult to string a sentence together when I feel under attack... she was very good at debating in school and so my DM encouraged her to argue whereas I was never encouraged to learn that iyswim. I think my DM was embarrassed in front of my DP so rather irritatingly apologised with a tinkly laugh for 'my daughters and their bickering'. I tried to say well wtf - there was no "bicker", I just got shouted at over a perceived facial expression, I didn't get a single word in and she stormed out while I stood there literally gaping. My DM did admit the next day that she can see that my sister's behaviour towards me is 'semi-abusive' but she doesn't know what to do and she tries to ignore it because we're generally such a close family who get on very well.
Other things she does include:
- any time we go out somewhere (shopping, to a restaurant, family event, whatever) with all or some of our immediate family, she makes every effort to 'freeze me out' of the event. It sounds so silly I know... examples include: 1. physically blocking me from speaking to other family members at a table by turning her entire body in our way and talking over me when I speak or snapping at me if I say 'oh can I just talk to X for a second'. 2. Taking issue with something small about me and saying it to every sibling/parent present. Recent example is 'I cant stand the sound of her high heels on the ground, can she just walk behind us'. And if I speak either a. Shouting over me, b. Negating what I've said and saying I'm stupid (even if the point she then comes up with is literally what I've just said) or c. Rolling her eyes (the least hurtful one really!) It just makes me feel a bit shit because I feel like if I point these issues out I'll sound overly paranoid or I'll cause a huge confrontation and be part of the problem, yet my other option (and what's gradually happening) is that I just talk less and less in these situations when she's there and go quieter. So I feel like I can't really be myself around my own family. I'm the only family member she treats like this.
I suppose with the above things I might think ok she's just a volatile argumentative person. But I'm the only person she treats like this. To our brothers and other sister, other than the very occasional sibling-scrap, she shows respect and allows them to have 'banter' with her, whereas the slightest joke from me often just provokes a tirade on how stupid I am. Worse still, she literally waxes lyrical to me and my parents about how amazing my siblings and their other halves are whilst she ignores the existence of my DP and once on a drunken tirade told me that we were 'disgusting' 
and tries to ensure that he is out of his depth conversationally when the family is together as she holds court conversationally and just brings up 'in jokes' that he wouldn't understand.
She sneers at me and my career. I'm by no means rich but I love my work, work hard, make a decent living and DP and I live within our means. Im just not a 'big house and fancy car' type of person. But there are constant remarks about me being a loser, a 'broke bitch', not having a job (despite the fact that I do 
not being cool etc. My siblings and I have all worked really hard in varying fields and we're always interested to hear about each other's work but then she ridicules mine to such an extent that I feel it tearing my confidence away... my field requires a lot of self-esteem and motivation (don't want to specify what it is and out myself further!) and I actually have worked on building up my confidence over the years and every time I see her it's like the wall is just knocked down again. And I just think why do I allow this?
Then, bizarrely, there are episodes where she'll be completely OTT affectionate to me. Eg drunk texting me how much she loves me and is glad I'm her sister etc, crying to me every time a boy upsets her, randomly asking me if I love my DP more than I love her. She has also opened up to me about her own issues and I really want to be there for her and have talked alot of things through with her. If we're having a heart to heart and she says eg 'whatever you might think, I do love you so much' and if I say 'well I know but when you did X recently it did actually upset me' she'll then say I'm crazy, playing the victim, what I'm saying never happened etc etc.
She very often asks me for favours; to post her things she likes from the country where I usually live, or driving her around to places when we're both in UK as she can't drive and it's 'too much' for her to get herself to certain places. Most recently she asked me to come and provide my work for free at an event she was doing at university so I agreed. In the time since I did this there has been the usual treatment and I just feel like such a mug for doing things for her that I would for my other siblings when she has just decided that I'm not worthy of respect. I have thought that I probably have to start saying no as a rule when she asks me for things. It's going to be difficult but it may be the best start.
So if you've read this far thank you soo much and you deserve copious amounts of 
!!! If we weren't a close knit family and my family life wasn't so incredibly precious to me I'd probably avoid her at all costs and just not visit. I mean, if she were a friend I think I'd just go NC. But my time with my family is something I look forward to the whole time I'm not with them and I just cherish them so much. I don't know what I've done to be pushed out like this and I sometimes wonder if I'm going crazy and it's normal sister life, but she just loves our other DSis so much and our DBs too. It's been like this with her for about the past 5 years, but when it started her moods with me were so much rarer, and it has really increased to where we are now, with me on eggshells and every word I speak in the home being either sneered at or shouted at. I just don't know what solution there could be to tackle this behaviour without upsetting the dynamic amongst the rest of the family and descending into us actually just 'arguing'. If that makes any sense.
I'm sorry this is so long. I could just do with any outside perspective because I just feel quite on my own with this.