-By Doris Toumarkine

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Arriving on waves of good buzz from the Toronto, Sundance and Tribeca fests and filmmaker Lynn Shelton’s success with her previous Humpday, Your Sister’s Sister establishes Shelton as a top-tier indie who delivers the quality goods that smart young audiences want to see. They’ll want to see this one and spread the word.

Shelton rounds up a great cast and gives them room to improvise. Well-established and rehearsed situations inform a nifty narrative concept: A rattled guy and gal experience a spontaneous and unlikely alcohol-driven one-nighter that they must keep secret from the gal’s beloved half-sister who unexpectedly arrives the following morning.

The film begins at an event commemorating the untimely death of slackerish Jack’s (Mark Duplass) brother Tom the year before, whose passing has devastated him. Tom’s friend Al (Mike Birbiglia) gives a toast that sets Jack on a rant. Noticing how damaged Jack is, his friend Iris (Emily Blunt), who had been Tom’s girlfriend, suggests he take a break to her family’s woodsy island retreat to chill.

Jack accepts, expecting to find quiet and solitude. Upon arrival, he discovers Iris’ older half-sister Hannah (Rosemarie DeWitt), recovering from her own trauma: the break-up of a seven-year lesbian relationship.

That night the two start sharing their stories, attack some wine to loosen up, vent their grief and end up in bed doing the deed. The next morn, Iris, who happens to have a thing for Jack, arrives unexpectedly and the game is on. Jack and Hannah, who is very close to her half-sister, work hard to keep their secret. Of course, they’re not successful, but yet another secret emerges.

Your Sister’s Sister keeps us guessing all the way. (How will Iris find out about what happened? Has Hannah with her Jack fling really gone over to the other team? Will Iris get Jack?) As this folie à deux escalates to maybe managing à trois, it provides a wonderful and often funny emotional ride. Duplass, again showing he’s got the comic chops and precision timing, deserves much credit as the wobbly and well-meaning hypotenuse in this unsteady triangle.

Also benefiting from a fine score, lovely rustic locations and solid production values all around, Your Sister’s Sister works as a convincing example of how a low-budget filmmaker—with the right story, cast, emotional truths, etc.—can get terrific work up on the screen that keeps costs down and viewers glued.