This fanciful biopic casts light on Mozart’s older sister Maria Anna “Nannerl”, a fine singer and instrumentalist in her own right whose ambitions naturally took a backseat to the boy wonder’s prodigious gifts. Based in part on the correspondence of their demanding father Leopold Mozart, the account is a quintessentially French one set in the 1760s when the children are aged 10 and 15, following the imagined events that unfold during performing tours to Paris and Versailles.

Nannerl’s journey centres on two fictionalised encounters with French royalty. The first, with the cloistered, illegitimate 12-year-old daughter of Louis XV, echoes the tragedy of her own thwarted potential. As Louise de France, Lisa Féret is blandly benign and monochrome, making it difficult to care about the rapport between the two young girls, who are in fact real-life sisters.

Meanwhile, the teenage Nannerl’s sexual awakening becomes a focus with the help of the brooding Dauphin’s smouldering gaze (Clovis Fouin). The attraction is inextricably linked with his intense admiration of her music, freeing her creative spirit – temporarily, at least. She is forced to dress as a boy in order to consort with the prince in public, but this narrative tool, again designed to emphasise the constrictions of gender, borders on the ridiculous: it’s hard to believe their meeting could have come about in this way. 

Family drama thrives at the heart of the story – a fitting theme for a film with the director’s daughter Marie in the title role. Nannerl’s secret resentment of Wolfgang is portrayed with subtle restraint. Leopold (Marc Barbé) dreams big for his son even as he relegates his daughter to mere accompanist and forbids her to play the violin or compose: pursuits deemed unbecoming for women approaching marriageable age. Her quest for approval and fatherly love threatens to extinguish her considerable talent, bending her into the Enlightenment’s feminine ideal of self-sacrifice and modesty.

Insights into the pressures of the Mozarts’ life on tour in Europe are set against touching moments of spontaneity not far removed from a normal childhood: Wolfgang and Nannerl clown around, improvise tunes and then race to the clavichord to bring them to life. Even in this sympathetic setting though, the famed child composer very nearly steals the show – David Moreau is pitch-perfect as the arrogant, impetuous young virtuoso already carrying the weight of greatness on his shoulders.

Sumptuously costumed and filmed in candlelit splendour at the palace at Versailles, Mozart’s Sister satisfies as a period piece. One of the film’s principal pleasures it the lush score by Marie-Jeanne Séréro, a pastiche of Mozartian style approximating what Nannerl’s abandoned music might have sounded like in full bloom.

At times the naturalistic editing feels clunky, as in one awkward scene when Wolfgang discovers his sister listening in on a counterpoint lesson barred to her. But nothing is more clunky or awkward than the cobbling together of history and fantasy. References to the young woman’s lot in life are heavy-handed for the most part, while one inaccuracy in particular rubs me the wrong way: the Dauphin asks Nannerl to sing the high soprano passage from the Allegri Miserere (“of course I know it”, she declares), but the piece was not heard outside of the Sistine Chapel until Wolfgang himself famously attended mass and transcribed it in one hearing, at least three years after the events of the film take place. The music convinces; the story does not.