Ah.... I'm sorry for not replying sooner, Silkthread. I've had some things to deal with. I'm also sorry for questioning you earlier, I was hurt.
I hope your sister gets better.
First of all in case you're blaming yourself, you shouldn't. I won't pretend to understand everything you might have gone through but I have also had to deal with a person who was impossible to handle. It's just hard to imagine dealing with someone sucking the life force out since as far back as childhood. One-sided relationships seem to work that way. The person repeating the same loop over and over like a broken record won't help things, either.
I'm trying to imagine how I'd be if my own sister was that way. Unlike with my mother I'm on ok terms with my sister. I see her maybe once a year or so although I can't say I'm always that happy about it. If she needed help and didn't have anyone else to look after her I'm not sure what I'd do. I'd probably help her out like you did out of some sense of obligation, but I can't imagine doing it forever. If she died, though, I would feel a part of me was gone as well.
I don't want to dig too deep if the subject is sensitive but I still might, accidentally, so bear with me. Also despite your sister being in hospital I would like to give you my honest opinion and I hope you can appreciate that. I don't know your sister so I won't be able to give anything beyond that. You say you've become apatethic to your sister's needs. I understand there are people who simply won't listen to anything, and borderlines are difficult to deal with as they enjoy feeling sorry for themselves.
Did she ever have anyone else to talk to? A professional? If her problems stem from home then I imagine you are likely to understand her problems and as such in a way I can understand why she would confide in you. I'm not implying you are responsible for dealing with her problems, though. I don't think you are.
You say you've closed your sister off. That makes me wonder about whether things used to be different between you two. She must have had her moments, like all siblings do. I know what it feels like helping someone out out of habit. I also know how it feels like getting paid back nothing in return -- despite your best efforts. I imagine it must be hard to deal with a person whose personality changes constantly depending on the trigger.
Self-loathing is also hard to deal with. People always need to be convinced they're good in the inside, that they somehow amount to something just by being who they are. If a person is never loved they will never learn that. It seems to be quite common among children with insensitive parents or parents with overly high expectations.
I've never had to deal with a person suffering from BPD. You describe it as being with a stranger. I think I can understand when you say you felt like the little sister. If that was the first time in a long while she tried to help you out of her own volition (out of her character as she may have been), then I wonder how you felt about it.
I would personally probably be angry if my sister started talking to me about forgiving my mother, especially since she was always treated better. I can sense you weren't too happy with what your sister said either. I'm just wondering how much you two talk about your parents. I personally haven't talked to my sister about my mother in a long time -- the last time was when I was kicked out. I think it might be better that way.
I understand the futility of trying to tell someone to keep still or try to reason with an impossible person. I mentioned I used to know someone like that, and I tried every way possible to get through to that person but it was impossible. Nothing I said registered. There was always some unbelievable reasoning that always linked to the person's misery.
It was interesting to read through your story. Take your time, I'll read your next post once you have it ready.