Reading scene in the kitchen

Early in the movie, there is a scene in which in Harry's sister Maria (Holly Aird) is sitting in the kitchen with Eva and her younger sister Janie. She reads a couple of things out loud and it is obvious that she is marginally literate and is only just now learning to read, apparently being taught by Eva and Janie. It is never explained why Maria can't read very well - she is not slow or mentally handicapped - and both Eva and Janie, who is about 10 or 11, can read normally. Does anyone know why this is the case? Is it something that was explained in the original story but not in the film? Perhaps it was not unusual for someone, particularly a girl, to have not received much schooling in rural England in the 1940s (when she would have been a child).