earning about five different novels written about Nannerl Mozart, I was able to track them all down, mostly through the secondhand book source, alibris.com. I chose one at random recently to read: Mozart's Sister, by A.M. Bauld.
This is a very strange book. My husband described it as "like a nightmare." It has a bizarre dreamlike quality. I would say it has only the slightest connection with Nannerl Mozart's real life. The narrator is Franz Xaver Mozart, the younger son of Wolfgang, who is visiting his aged, infirm aunt Nannerl, as he actually did. This seemed at first to be a similar format as Mozart's Last Aria, however, this Franz Xaver expresses disgust with his decrepit aunt, and, in a science fiction twist, he can transform himself into a spider (shades of Kafka!) and travel back in time to observe his family before his birth. Other historical events that appear in this novel include a visit by Vincent and Mary Novello, who did in fact travel from England to visit the elderly Nannerl and interview her about her life and her famous brother. And, like both Mozart's Last Aria and Marrying Mozart, letters are very important in advancing the plot. The Mozart family is, of course, known for being prolific letter writers.
The author invents several important characters who have significant interactions with Nannerl and Leopold Mozart, including a love interest for Nannerl. Bauld does include an Author's Note clarifying which characters were invented, but these characters add to the strangeness of the mood and the depiction of the real people. Most of the characters seem to be unhappy. Example (and spoiler -- alert!): Nannerl love interest is an itinerant draper and amateur musician who is Jewish. They fall in love, but neither tells the other, and after a series of unhappy misunderstandings, he eventually leaves, possibly to perish in a shipwreck on the way to Van Dieman's Land (Tasmania). But maybe not. maybe he made it up.
I was very confused by the end of this book. I think I will go back to nonfiction for awhile.