The film is scheduled to open Friday at AMC Quail Springs Mall 24, 2501 W. Memorial.

Written and directed by Lynn Shelton over 12 days, Your Sister’s Sister is another indication of the ongoing mainstreaming of “mumblecore,” a quasi-film movement punctuated by improvisation, modest production values and, all too often, amateurish notions of storytelling.

But Shelton, who in 2008 helmed the critically acclaimed Humpday, is no navel-gazer. This movie is smart, funny and absorbing, and it benefits from a terrific acting turn by another mumblecore alum, Mark Duplass (TV’s The League), who has directed several nifty movies (The Puffy Chair, Baghead, Cyrus) himself.

Duplass gives one of three strong performances, joined by Emily Blunt (The Five-Year Engagement) and Rosemarie DeWitt (TV’s United States of Tara) for what essentially could be a three-character stage play. What the movie lacks in a dynamic visual style — despite its setting in the scenic Pacific Northwest — is made up for by keen observance and emotional honesty. There is hardly a false note here.

At the story’s center is Jack (Duplass), an underachieving 30-something filled with rage, self-absorption and grief over the untimely death of his brother. Iris (Blunt), Jack’s best friend and the ex-girlfriend of the deceased brother, urges him to seclude himself for a couple of days at a cabin her family owns on a nearby island. He accepts the offer, bicycling to a ferry and heading to the island for “some head space.”

But the place is already occupied by Iris’ lesbian half-sister, Hannah (DeWitt), who is seeking alone time after the end of a seven-year relationship.

“I apologize if I’m barging through the doors of your privacy,” Jack tells her, shortly before the unlikely pair share a bottle of tequila, some drunken conversation and — in a funny and awkward why-the-hell-not moment — a bed. Clearer heads emerge the next morning, and Jack realizes he really doesn’t want Iris to know about the onenight fling.

And then Iris shows up for a surprise visit.

Some twists are expected, some not, but it all feels authentic and urgent. The three principal actors are outstanding. Duplass imbues Jack with a sharpness and wit that make even the character’s obnoxiousness endearing.

Just as good is DeWitt, whose humor and screen presence are as commanding here as they proved in 2008’s Rachel Getting Married. Blunt, the only marquee name here, fares less well, but she’s also saddled with a passive role that just isn’t as juicy.

Shelton wisely keeps directorial intrusions to a minimum and lets her cast do their thing. The result is a first-rate dramedy that observes, and ultimately celebrates, the heart at its most flawed.

“I’m emotionally, at best, precarious,” Jack tells the women at one point, “at worst, a cripple.”

Hey, we’re only human.

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• Cyrus film review 
• Humpday Blu-ray review  
• The League: The Complete Season Two Blu-ray review   
• United States of Tara: The Second Season Blu-ray review