My sister doesn't remember anything either, in fact she denies anything happened. She has grown into a very nice (to others) full functioning adult that is a great mom and wife. She had to do this to survive our life, I was told by our alcoholic dad's therapists.
I on the other hand remember everyday and even at the young age of three. Course I was beat and she wasn't. I don't know why I remember everything, my husband is amazed at what I recall. It is a burden to live with, but that's who I am. I have grown and gone beyond my childhood but it's very vivid still.