Last year brought a lot of chatter about cinema’s obligations to historical accuracy. Did Selma distort Lyndon Johnson’s role in the civil-rights struggle? Did American Sniper sanitize the Iraq War’s most lethal sharpshooter? Whatever the answers, we can conclude that the further we get from the historical period in question, the less discrepancies seem to matter. Which is why few people will fret over whether the historical characters in Beloved Sisters actually got it on as a threesome.

There’s no definitive proof that German writer Friedrich Schiller was snuggling up with his wife’s sister, but this movie certainly likes the idea. In the film, set in the late 18th century, Schiller (the callow Florian Stetter) meets future wife Charlotte (Henriette Confurius) when he is still a threadbare playwright. Charlotte has the luxury of marrying for love, because her older sister Caroline (Hannah Herzsprung) has already married for wealth, thus propping up the fortunes of Charlotte and the sisters’ shrewd mother (Claudia Messner). But both sisters are close to the writer, and his erotic attention is clearly divided. Charlotte is less formed and apparently somewhat uncomplicated, but Caroline is a complex woman and a talented writer herself. Schiller publishes her serial novel in his magazine, a story that becomes the talk of the literary world for a few months. The casting itself tips the balance in Caroline’s favor: Herzsprung, an actress of hooded eyes and smoldering demeanor, is a richer performer than the out-pointed Confurius.

With its heavy-breathing material, Beloved Sisters has possibilities, but veteran director Dominik Graf swerves recklessly between the arthouse and soap opera. The thing stretches out to 170 minutes, which makes for a lot of pretty costumes and houses but not much momentum. I ended up enjoying the movie, in part because Graf arranges the entire story around letter-writing. He’s surely cribbing from Francois Truffaut’s Two English Girls, another love-triangle period piece that featured actors addressing the camera as they narrate their passionate letters to one another. The device gives Beloved Sisters an antique quality that lifts it from the humdrum realm of the average miniseries.

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BELOVED SISTERS Runs Fri., March 13–Mon., March 23 at Grand Illusion. Not rated. 170 minutes.