Nannerl Mozart (Marie Féret) in the film ‘Mozart’s Sister’ by by René Féret. (Sylvain Bonniol)

Biopics are never easy. The historical record can be unreliable or can yield conflicting information. Groups with vested interests may pressure a filmmaker. But writer-director Rene Feret faced a different hurdle when making the French-language feature “Mozart’s Sister,” which opens Friday.

His challenge was to create a compelling movie about a woman whose legacy did not survive her. That woman was Maria Anna Walburga Ignatia Mozart, better known as Nannerl, the beloved elder sister of Wolfgang Amadeus Mozart and a talented musician in her own right.

Nannerl, a child of the 18th century who lived well into the 19th, was a prisoner of her time and her sex. Women of her era did not have careers playing the harpsichord — on which she was a virtuoso — let alone make their mark as composers. And though letters from her brother suggest that she was talented at writing music, none of her scores survive.

The paucity of historical evidence attesting to Nannerl’s gifts left Feret with limited options. How do you believably reconstruct a life when the materials used to measure achievement no longer exist?

To balance verisimilitude and a compelling narrative, he chose to meld fact with fiction. “I realized what I needed to do was not base a film just on the letters that survive, but also to invent a story that would present us with the grandeur and decadence of this period,” Feret said, speaking though an interpreter by phone from France. “So the film is 40 percent reality and 60 percent fiction.”

He decided to focus on Nannerl’s family life as a way into her story, paying particular attention to her complicated relationship with her famously exacting father, Leopold, who first encouraged and later discouraged her musical progress. “I am the father of two daughters, so I projected myself into that situation and wondered what it would be like,” Feret said.

In fact, he took a step beyond wondering and cast his elder daughter, Marie Feret, in the role of 15-year-old Nannerl. Though the actress expresses ambivalence about a career in movies, she has strong feelings about her character and the societal restrictions imposed on her.

“She was born in the wrong period,” she said. “I think Nannerl could have been as famous as her brother was.”

Perhaps the biggest challenge the filmmaker faced was how to present Nannerl’s now-nonexistent music. For that, he enlisted composer Marie-Jeanne Serero. “The challenge I gave my composer,” Feret said, was to “write music as if she were her.”

Serero, who besides composing music for theater and film also teaches at the Paris Conservatoire, reached into her academic background for inspiration.

“My approach was to try to see what kind of music the Mozart children might have been exposed to,” she said. “Leopold would have given them musical exercises and some knowledge of counterpoint and harmony. Certainly they must have heard Bach. I tried to use this as the basis for imagining how Nannerl’s musical life would have progressed — how she would have had some training at the piano, some violin lessons, some voice lessons. And I wanted to use all of this as a basis for what her music would be like.”

Yet for all her familiarity with the period, Serero ultimately felt the need for poetic license. “When you listen to the concerto she has composed, you can hear echoes of Bach and of Mozart, but also a foreshadowing of Beethoven. Of course, it would have been impossible for her to compose in this emotional, Romantic style. Perhaps this was a mistake on my part, but it was very truthful to her character. So I took a chance.”

The toughest part of Serero’s job was distinguishing between Nannerl’s compositions and the film’s underscore, which she also wrote. “The background music really reflects Rene’s aesthetic,” Serero said. “I wanted it to be an accessory, like a scarf. I also allowed myself to go further into the Romantic period with that music. . . . The orchestration is bigger in the background music; we’ve moved beyond the clavichord and the small string orchestra to a fuller orchestra.”

Just why Nannerl’s story was so important to Feret is something even he has trouble explaining. Part of his interest, to be sure, was in taking a stand for someone who had no way of asserting herself. “I really believe — and perhaps I’m mistaken — that Nannerl had a potential that was much greater than her very short career allowed her to realize,” he said.

But there’s more to it. “You know, it’s almost like asking about falling in love,” he said. “Sometimes it just happens, and you don’t know why. Now I realize that a number of the themes were very important to me: the father-daughter relationship, the idea of creativity and creation, the question of identity, the idea of a lost life, and also the history of women. For all these reasons the film was important to me — even if all that was perhaps not evident right at the beginning of the process.”

Mermelstein is a freelance writer.

Mozart’s Sister

opens Friday at Landmark’s E Street Cinema, the Shirlington 7 in Arlington and Cinema Arts in Fairfax.
120 minutes. Unrated.