I think your sister has decided to remember her past differently, which is why she is acting as if nothing is wrong with your mother. A lot of children of alcoholics will do this to deal with the underlying pain of the truth. My father turned me into a paranoid, he acted as if there was a big secret in the house and demonized the police, he would react very angry if we had to call the police. That is the truth, but very seldom do I try to remember the past like that. I am sure your sister knows she knows the truth but whenever it try's to pop in she probably pushes it out of her mind. She could be trying to find herself as well, having identified with a parent who is no longer around, a person can get very lost, I know this as I lost my mother in 2011, It is a very confusing place to be and we all react in our own way.

Hope that helps!