I'm enjoying this book a lot! As a classical music fan and musician I'm always interested in accounts of composers and their lives. It seems that there is a big trend of books right now - so & so's sister, wife, daughter etc., just as we are defined a lot of the time by our male relation. I never even knew Mozart had a sister, and even though it is a fictional account, I'm looking forward to reading the book.
I discovered a book of letters between Woflgang and his sister while he was in Italy with his father "on tour". He addressed them lovingly to "Dear Horseface". I'm sorry I can't give any further information but if you can access the Muhlenberg College Lib'y in Allentown PA their Refernce Dept may be able to locate it in their holdings. The auuthor of Mozart's sister may also be able to provide info on this resource.