Hi Fergus. Have been following the thread but not had time to post again. That was a nice last post from you, good to hear something positive is coming out of all this heated debate! (I do agree there's probably quite a lot of projection here!)
Anyroad, there's nowt wrong wi opening a can o worms! Better out in the light of day than festering away inside...
My feeling now, having had so much more information, is that you're actually stuck in a co-dependent relationship with your sister. You've taken on responsibility for fixing her, and she's largely given that responsibility over to you too. And from that perspective, I think the best thing you could do would be to shift your focus away from her and her issues and to you and your own issues. Open that can of worms properly and have good dig around, whether that means counselling/therapy for yourself or some kind of support group like CoDA (Co-Dependents Anonymous), for example.
I imagine that kind of stuff is pretty alien to you as you've always been the strong one, the coper in many ways - but it actually takes a different kind of strength to confront your own fears and hurts, and to feel real compassion for yourself and what you went through/are still going through as a result of this very, very warped family dynamic your parents created. (Self pity is such a loaded term with such negative connotations, but actually feeling compassion for yourself is a very positive and healthy thing to do.)
You never know, if you take on some of the mantle of the "broken" one by exploring your own hurts, it might actually free your sister up to step into the coper role a bit and be more pro-active in sorting her own life out. We really do get shoved into these roles by our dysfunctional families, and you can't blame your sister for not being able to step outside her victim role if you're similarly not able to step outside the coper role and take on the victim one to an extent yourself. Does that make any sense to you?
Not that that gives your sister a free pass to treat you as badly as she has done on several occasions, from the sound of it. It's almost like she's in turn scapegoating you, and you don't deserve that, any more than she deserved to be scapegoated by your mother.
Also, if she continues to take her stuff out on you, it's like you're her safety valve, a way for her to cope with things without actually having to address the underlying issues. As well as being really awful for you, this could actually be stopping her from moving forward. The move to take control over whatever she can in her own life has to come from her, no one else can do that for her, and if you can find a way to still be a loving presence in her life while no longer tolerating being used and hurt yourself like this, she might find it in herself to take that decision to move forward for herself, to life for herself.
I know you must be tormented though by the thought of "what if it doesn't work like that", and I can see you really are between a rock and a hard place. But that brings me back to getting some help/support for yourself; that in itself can in no way be seen as abandoning her. Finding and healing the victim in yourself can only be good for both of you.
There was a lovely thread a while back by a woman whose mother had similarly favoured her over her sister, and while her relationship with her sister was positive, she found it affecting her relationship with her own DC, in that she was repeating the past by favouring one of her DC over the other, and was devastated to acknowledge that.
Obviously the parallel here is slim but it's just that this woman was not someone who would typically seek counselling, she was the one for whom life had worked out well, while her sister really struggled - but when she did do so, it was immensely helpful to her and she was able to really connect with the DC she'd previously felt no love for. Counselling is by no means a cure all and you have to be choosy about finding the right person for you, but it really can turn things around sometimes. In your case the very act of saying you are also damaged by this, you deserve support with this, could be very powerful indeed.