Jack (Mark Duplass) is shell-shocked and angry from the recent death of his brother. His compassionate best friend Iris (Emily Blunt), whom he secretly pines for, sends him to her father’s secluded log cabin, to regain composure. When he arrives, he’s surprised to find Iris’s sister Hannah (Rosemarie DeWitt) already occupying the cabin. She’s in recovery mode, too, a lesbian still hurting from a bad breakup; unable to sleep, they bond over whiskey shots. Drawn to each other’s endearing insecurities, they succumb to a careless one-night stand. Complications ensue when Iris shows up the next morning, intent on telling Jack that his feelings are in fact requited.

Lynn Shelton’s “Your Sister’s Sister” is a considerable improvement over her last film, “Humpday,” which was also admirably naturalistic but confounded by a high-concept, one-joke premise. Most films in the so-called mumblecore movement play out like mere “what if” drama class experiments (“Humpday,” for instance, attempts to answer the question, “What if two neurotic straight guys got roped into shooting a gay porn film?”) Films like “Humpday” and “The Freebie” are typically more clever than funny, with actors that are low key to the point of inertia (most of the dialogue is stammered and mumbled) and shaky, amateurish camerawork meant to lend a documentary feel.

“Your Sister’s Sister” is certainly small-scale and stagey—almost all the action unfolds in the cabin, and there’s little to no commentary on outside life—but its story is more heartfelt and recognizably human than that of “Humpday.” The screenplay (also by Shelton) perfectly captures what’s gently charming, even attractive, about low self-esteem (“Emotionally, I’m at best volatile and at worst crippled,” Jack laments about his lack of maturity). There are plenty of insightful character details, such as the otherwise nonchalant Hannah’s anal-retentiveness about her strictly vegan diet, or the way Jack and Iris are so comfortable together, yet so afraid to overstep boundaries, that they sleep in the same bed facing opposite directions. There’s even some scenery here—the gorgeous lake and forests surrounding the cabin make you want to relocate to the earthiest stretch of the Pacific Northwest, to meet warm characters such as this.

Duplass (who came up with the initial idea for “Sister”) has grown as an actor; though he previously only revealed one dimension—the cuddly shoegazer—he now exhibits the full range of man-child emotions: petulance, self-doubt, panic and the need for acceptance.

But the real standouts in “Your Sister’s Sister” are Duplass’s two female co-stars. The mumblecore scene has begun to attract A-list actors, and Blunt absorbs Shelton’s laid-back, improvisatory technique with remarkable ease. In the past, Blunt has proven equally adept at playing brittle bitches and doe-eyed ingénues, but approachability is not a word you’d use in the same sentence with her; here, she turns down the glamour and gives a relaxed, down-home performance. Goofy and self-contained, she embodies everything that’s alternately frustrating and adorable about platonic female friends, whom you desperately want to date.

And DeWitt shows you everything that might be sexy about a slovenly-dressed, slightly past-her-prime single woman. With her beak-like nose and intense stare, she’s a natural seductress, and from the quirky way she tosses off a line—slightly aloof yet full of self-doubt—you understand why Jack can’t resist Hannah, and why the flightier Iris looks up to her. It’s a marvelously intelligent performance.