I find it difficult to believe that any of Mendelssohn's accepted music was written by Fanny because their geniuses were so different--her sense of form is much more elusive, almost Chopinesque. Mendelssohn's form is more like Beethoven's, where you always know precisely where you are.
Rumour has it that some of Felix Mendelsohn's pieces are actually by Fanny, as she was not permitted by the family to engage in professional musical activity, so her brother allowed her to publish some works under her name. Maybe some of you have more specific informaion about this intriguing subject....
I'd like to slap Clara's Fanny! (does that answer your question?)
In fact there was a woman on the show talking about a book she read - i think based on Clara Schumann's life, (and Fanny is one of the characters in it) it might have been called "Clara" - has anyone heard of this or read it? I would love to get hold of it...will go and look on google...!