Tangentially, DSis fairly recently (a few months ago) decided to take responsibility for something that I suspected, knowing the other individuals involved, was likely to get very tricky and end up being hard work and causing her major stress. I said, when we talked about it, that she didn't have to do this thing if she didn't want to. She didn't really want to do it but she felt she ought to. I said what I'd say to anyone else — there is always a choice and no one would think any the less of her if she decided to say no
If she does have EUPD then at a guess, based on what you say, she felt she ought to. A morals-related reaction - 'ought', 'shouldn't', 'what a good person does, what a bad person does' which is a fundamantal black/white thinking - few shades of grey, which is what really exists in most people. So, a sort of internal pressure placed on herself to do what she thinks needs doing, and perhaps what she instinctively thinks a 'good' person should do.
You then said 'no one would think any the less of her if she decided to say no'. It's possible that she reacted very badly because there was a respect-dimension - people thinking more, or perhaps less of her - if she is very fragile in the ego region and highly sensitive to rejection, this may have 'triggered' her. Im starting to dislike the overuse of the word triggered, but it does perhaps fit here.
If this is accurate, then if she's that fragile then she has little fundamental sense of self-worth and little ability to kind of draw a line and say that other peoples' reactions and opinions of her don't matter. She has no control over them (and shouldn't have) but others' bad opinion of her really devastates her.
The things you're saying are reasonable and correct, but she may need professional help that is detached. As her sister you may be too close for her to listen to, specially if she blames you.
There have been times when she's really angry with me that I've felt that I'm dealing with a child having a tantrum. There's a different quality to things. Beyond communication.
Familiar.
I'm scouring my memory for anything that might have happened in our childhood to traumatise her and I'm coming up with nothing.
I'm afraid that -if- this is trauma-related EUPD then it's possible that may literally have nothing to do with your family, and in fact she herself may not know. As I say, I've dealt with someone else who had serious EUPD and the cause was discovered much later and by chance. Bloody tragic situation and the victim will never be 'normal'. It's also possible that there were dynamics in the family that you were not wholly aware of; parents can react differently to one child than to another sometimes and if there was already a form of trauma and there were subtle differences, they may have unfortunately combined and so you, her sister and brother, get the effect as her nearest.
Does she have an alcohol or drug problem? People with untreated EUPD often have an addiction problem. It's self-medication.
I'm hedging a lot of what I say again because of the online thing, and please note, I'm not medically trained.