Chillout, I LOVED the part of your post that says she doesn't just have issues, SHE SUBSCRIBES.

LOL. It sounds like your sister is looking for some sort of validation of what she went through. She doesn't understand why none of you don't tell her she is right and keep on telling her everytime she talks about it. I definitely understand your frustration.
I was in the same situation with my brother when I was around your age. It is better now, as we have gotten older and our parents have died. But, one day, when he was going on about it and asking me questions and basically, obsessing as your sister is doing, I asked him why do you keep rehashing these terrible memories? What purpose does it serve? It only makes you very unhappy and it brings everything back up and makes the hurt new all over again. Why are you doing this to yourself? Do you love your family now? Spend your time living in the here and now and enjoy your children. Work on being a better parent than what we had. We can't change what happened. Our parents made A LOT of mistakes. We can't change, we just have to accept it and use the knowledge that we gained from that not to make the same mistakes with our children and each other. Spend time with your family and work on making your kids into good and productive adults. Don't make your problems into everyone else's problems because that negatively just makes people want to push you away. I told him I didn't want to be around him because of it. That I was focusing on my life now and enjoying it.
If you can do this with her, and it doesn't make her stop and think, I would suggest that she go into counseling to find a productive way to work through her anger issues with your Mum. Tell her she deserves to be happy, not miserable and she needs to find a way to move past this. She needs to understand that your other sister's health is more important than her hurt feelings from the past. Then, if she continues, you can say we have already had this discussion, you know exactly how I feel about it and what I think you need to do. And, change the subject.