Just ran into an article that talks about Mozart's older sister - Maria Anna Mozart, nickname Nannerl, - older to him by about 4.5 years.
Maria Anna Mozart as a child (1763) (portrait said to be by Lorenzoni)
"We have few certainties regarding Nannerl Mozart’s musical aptitude but they are significant. We know that she was considered a virtuoso on the harpsichord. As a little girl she performed as a duo with Wolfgang in the international tournées organized by their father, and in the announcements of concerts and newspaper articles she was named – and praised – first. ............. In short, there are many letters in which Herr Mozart vaunts his daughter’s skill at the keyboard and the praise she received. .............once she had grown up and was therefore no longer ‘usable’ as a child prodigy, would be to make her teach the harpsichord, so that the money she earned could go to fill the family coffers and finance the study, travel and artistic promotion of her brother. Naturally Nannerl continued to play in public, but mostly – and not by chance – she performed her brother’s compositions.
To conclude, we can be certain of one thing: due to circumstances, over the course of the years Nannerl Mozart’s musical talent, however great it was, gradually dried up and went to waste"
In addition to Rita Charbonnier's book, it seems there is also a recent movie about her - albeit a fictional version of her life. And Wikipedia may have some more "real" details about her.
Unfortunately, there is no surviving records of any compositions by Nannerl but so it goes... here then is music by her illustrious brother -- even in a post about her, music by Wolfgang is what will have to be heard here too! I really love almost all compositions by Mozart and really cannot pick a favorite to highlight today - so this is just a random pick!
Music for the day: Mozart's Piano Concertos 17, 20, 21, 23 & 24, with Arthur Rubinstein on the piano with the RCA Victor Symphony Orchestra.