It sounds to me you are in need of closure. If for nothing else but yourself. Do what you need to do now, while you still can, to get the closure you are desiring. It's not like you can't go and tell her how you feel about what she did or did not do to you and your other sister. If you need to forgive her you can do that as well (forgiving does not necessarily need to mean forgetting). Go and have a conversation with her. Get it out so you can move on.
While it's awful what happened, it sounds like she was somewhat a product of her upbringing. Definitely not a valid excuse for the mistreatment of others though. I am glad you have been able to move ahead and become a good person in spite of your upbringing. It sounds as though your sister was not as strong as you were.