Unfortunately, Dizney loves to take childrens tale with a moral, and then dizneyfy it. The moral is twisted, and the effect..., I would avoid showing to any child (or adult for that matter).
The reason that her step mother and sisters didn't recognize her is because she is cinderella. She is called cinderella because she was always covered in ash (cinders) from the fireplace/stove. She was always a girl covered in dirt.
And the slipper is very important. In the story, the sisters want to be the princess so badly that one cuts off her toes, and the other cuts off her heels. But, the crow in the tree spouted, "...there is blood in the shoe, she isn't true"
And, at the wedding Cinderella's stepmom is forced to dance in red hot slippers. If you do not know what this is, watch "The Tenth Kingdom".
Ohhh :o . Is it a movie or a series?
The 10th Kingdom is a TV-mini-series, errr, its a really long movie.
http://www.imdb.com/title/tt0207275/