A childhood belief about family

My sister used to believe that my mother couldn't read.
She was obsessed with the story of "Cinderella" and wanted it read over and over again (literally. Finish and restart.)
Apparently, my mother got tired of it and told her she couldn't read. I don't know why she fell for it, as my mother had read it before. But I guess she believed it, as she couldn't either at the time.
She said that she thought my mother couldn't read until well into elementary school. What's even more funny is that she also told people at school her mother couldn't read. My mother was NOT pleased to hear this (and denies that she ever said it - but I believe it.)