Your Sister’s Sister

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“Your Sister’s Sister,” a funny, small-scale charmer with a trio of dynamite performances, plays out like a smart indie version of “Three’s Company,” complete with a guy named Jack. This Jack (played by likable everydude Mark Duplass) is a scruffy Seattleite still reeling from the death of his brother a year ago. His platonic best friend, Iris (Emily Blunt), orders him to clear his head by spending a weekend at her father’s isolated cabin on a nearby island, unaware that her half-sister, Hannah (Rosemarie DeWitt), is planning to use the place for her own emotional regrouping, having just ended a long-term relationship. Jack and Hannah knew of each other but had not met until now, and a late-night emptying of a tequila bottle helps them get comfortable sharing the house. Iris makes a surprise visit the next day, filling out the three corners of the awkward comedy triangle. Confessions, revelations, and misunderstandings mark the rest of the weekend.

Written and directed by Lynn Shelton — whose last film, “Humpday,” was just as truthful and funny — “Your Sister’s Sister” has a loose, improvisational feel that can be immensely satisfying to watch. The comedy and drama (mostly comedy) that emerge do so naturally, even though we realize that the circumstances have been contrived by a screenwriter. The warm chemistry among the actors feels like an invitation for us to flop down on the couch next to them and enjoy their companionship.

Of course, we get to enjoy it from an emotional distance, laughing conspiratorially as two sides of the triangle try fumblingly to keep secrets from the third. But once Jack, Iris, and Hannah have all made their discoveries — once everything hits the fan — the movie doesn’t quite know what to do other than find perfunctory resolutions. The fun was in seeing these authentic, believable characters squirm around the truth as they strive comically to avoid hurting one another. When it comes time for the frowny aftermath, Shelton’s heart isn’t in it.

And who can blame her? Nobody wants to see jovial folks like this deal with relationship crises. But all’s well that ends well, as they say, and it’s no spoiler to reveal that this astutely observed mini-farce ends in a way that leaves everybody happy, the audience most of all.

B+ (1 hr., 30 min.; R, abundant harsh profanity, some strong sexuality.)