I agree with Tatar that Cinderella and Donkeyskin stories should be studied together. They do have some similarities in the plot lines as well as the intricacies of the parent-child relationships. For example, 'Katie Woodencloak' starts out as a story similar to Cinderella, where the father is absent and the evil stepmother tortures her. However, the ending is more like the Donkeyskin stories. After the bull leads her through 3 forests, she becomes a scullery maid at a castle and meets the prince at 3 different balls. With her shoe, he finds her, and they go about their happy lives.
Here the story overlaps somewhat with both tales. Nevertheless, in both Donkeyskin and Cinderella stories, one parent is either dead or absent and the other (including stepmothers and fathers who want to marry their daughters) are abusive, cruel, or otherwise cause the girl to run away where she becomes "Cinderella" and eventually finds her prince. It seems fitting that they be studied together, even if they are kept in separate categories (as they should).