There's a nice, loose-limbed improvisatory feeling to "Your Sister's Sister," a quality that identifies it as the work of artisans, not assembly-line professionals. It's chatty rather than plot-driven, and its belly laughs are driven home more often by an eloquent sidelong glance than dialogue zingers. The story -- siblings Emily Blunt and Rosemarie DeWitt in an emotional/romantic tangle with aging slacker Mark Duplass -- is stuffed with insight and humor.
It's clear that Jack (Duplass) is in need of help in the opening sequence, when he punctures the mood at a one-year memorial for his deceased brother, Tom, by listing his many faults. To the rescue comes Iris (Blunt), Tom's perfect girlfriend and a dear friend of Jack's. She sends him on a solo getaway to her family's retreat in Washington's verdant San Juan Islands to get his head together.
There he encounters Hannah (DeWitt), Iris' acerbic lesbian sister, who returned unannounced to get over the breakup of her long-term relationship. Tequila and teasing lower their defenses, and before you can say blunder they're sharing the same guest bedroom. Dramatic algebra commands that Iris must make a surprise visit, inciting hasty alibis and evasions. With a minimum of fuss, writer/director Lynn Shelton sets in motion a course of complications, confusion and confessions.
The actors are delightfully naturalistic. Duplass, a talkative, cuddly type, has several reasons to be mortified if his 8-second lovemaking session with DeWitt becomes sisterly gossip. The gentle, sweet Blunt, whose feelings have migrated from the late Tom to his brother, turns to DeWitt for assurance that what she's feeling isn't delusional or disloyal. DeWitt, who is working an agenda of her own, has genuine feelings for both of the others but insists on feeding them her atrocious vegan, gluten-free pancakes. The film is warmhearted and generous to each one, and sympathetic about their dilemmas.
The film is too kind to deny the characters the reconciliation we wish for them, but too clever to tie it all up in a neat package. Even after the happy ending, they're dealing with a can of worms that's still comically wriggling as the credits crawl.