Can I use "of" instead of "from" here?
Excellent question. "Of" is used when talking about possession and "from" is generally used when talking about the origin, source or a starting point for something. In this case, the sisters are talking about the source of the ash that is on Cinderella. Because they are talking about a source, they have to use "from", it cannot be replaced by "of". Ex: The name of the movie. (The movie's title - possessive). Ex: The name from the movie. (A name which originated in the movie).