Your Sister's Sister opens in a living room crowded with people gathered to remember a dead friend. Photos are passed, glasses are clinked, eyes glisten with tears. Since this is the work of Lynn Shelton, the Seattle filmmaker with a gift for capturing the telling minutiae of everyday life and assembling it into a story, this living room feels like somewhere you've been, or might go tomorrow. (You've sat on the floor next to just such a radiator, rested your back against just such a sofa.) The plot kicks in with a toast given by a memorial attendee played by writer/storyteller Mike Birbiglia, who stands to share a tale illustrating the uniquely good heart of the friend who was lost. This toast is promptly shot down by the dead friend's brother (Mark Duplass), who holds forth, with dickish insistence, on the self-serving motives of good-hearted people and the treachery of reminiscence.
His buzz-killing transgression inspires a loving response from his dead brother's ex-girlfriend (Emily Blunt), who offers up her family's cabin in the San Juan Islands as a much-needed retreat for her stunted-by- sadness almost-brother-in-law. Duplass's character—Jack is his name—takes her up on the offer, and after a gorgeously gray ferry ride, he finds himself at a supposedly empty cabin that is not empty. Unbeknownst to Blunt's character—Iris is her name—the cabin is occupied by her half-sister, Hannah (Rosemarie DeWitt), who, after some explanation from the bumbling Jack, accepts this impromptu visitor into the cabin.
Thus commences Your Sister's Sister's uniquely complicated love triangle, which is explored with great charm and clarity for the duration of the film. To avoid spoilers, I'll just say it involves sorrow, situational heterosexuality, and the sometimes cruel and even criminal things we so blithely do to each other. Like Shelton's previous Humpday, this is a film that's made by its performances, and Duplass, Blunt, and DeWitt are all perfectly wonderful. Shelton's dialogue is resolutely conversational, even in the most heightened moments, and the way characters express themselves can be as revelatory as what they're saying. The best moments are the smallest: deeply connected friends trading embarrassing stories around the dinner table, midnight chats between insomniacs.
But then come the big moments—essentially, mumblecore spins on Major Plot Twists—and Your Sister's Sister loses its footing a bit. Whereas even the most outlandish twists in the would-be porn comedy Humpday felt connected to life as we know it, Sister offers a number of if-you-say-so moments, where characters behave in ways that aren't necessarily unbelievable but lack the deeply familiar humanity that gives such slice-of-life films their heart and power. Despite the occasional off-step, Your Sister's Sister marches to a satisfying and appropriately messy conclusion. It's a small, beautifully acted film that touches on a number of deep issues (grief, family, forgiveness) without succumbing to cliché. Go see it. 
