Maria Anna known as "Nannerl" is the first child of Leopold and Anna Maria Mozart. The Mozarts are a poor family but she and her younger brother, Wolfgang, are considered child musical prodigies and the ambitious Leopold is determined to parade them before the aristocracy of Europe because, in the words of Nannerl, her father is hungry for glory.
She loves her brother whom she considers a gifted idiot but to Leopold and Anna Maria he is an unparalleled genius whose career advancement must be the principal goal of the family. This is also the 18th century and women are thought to lack the prerequisite intelligence for musical composition. Furthermore Leopold prohibits her from playing the violin in which she excels because it is considered a man's instrument not appropriate for a female.
Frustrated by her father's indifference to her dreams Nannerl is pleasantly surprised when during the Mozarts visit to France she meets the young, erratic Dauphin, son of King Louis XV, who admires her musical compositions and encourages her to write more of them.
Nannerl had earlier been befriended by the Dauphin's younger sister, Princess Louise Marie, during a stopover at an abbey where the princess was residing with nuns at the request of the King whose sins were a major topic of the day and who enjoyed having his children atone for them. At first the princess views Nannerl as a convenient person to carry messages back to the royal court but in time she sees her as a soulmate separated by class but united by paternal misdeeds destined to eclipse both of their lives.
Mozart's Sister is a splendid piece of historical speculation and based on the few facts we have it's an entirely plausible story allowing of course for some dramatic license. Rene Feret's camera expertly takes us through this introspective character study backed by good production design and a fine cast including Marie Feret (the director's daughter) as Nannerl; David Moreau as Wolfgang; Marc Barbe and Delphine Chuillot as Leopold and Anna Maria Mozart; Lisa Feret (also a child of the director) as Louise Marie and Clovis Fouin as the Dauphin.