Notes from Leon Botstein's "Mendelssohn and His World,” edited by R. Larry Todd, 1991, Princeton University Press.; and the article "Mendelssohn: The Greatest Jewish Composer?" from the Jewish Standard, 10.23.09.
Mendelssohn’s beloved and loyal sister, a few years older, may have been as gifted as he was – both as a pianist and as a composer.
But Fanny didn’t enjoy any sort of career. In a famous and appalling letter, her father wrote that she must “prepare earnestly and eagerly for your real calling, the only calling of a young woman – I mean the state of a housewife.”
She did compose some music – 466 pieces – and Felix had some published under his own name (with her permission).
Years later, when Felix was in England playing the piano with Queen Victoria and her consort, Victoria played one of her favorite Mendelssohn songs, called “Italy.” Felix told her: Fanny wrote it.