Mark Duplass gazes at a far-away mumbleshore in Your Sister's Sister

A late arrival at the mumblecore party, Lynn Shelton established herself as a filmmaker to watch with Humpday (2009), in which two straight pals dare each other to couple on camera for an amateur porn video. That film ultimately collapsed under the weight of its dubious premise, but in this one the center holds despite a comparably far-fetched story line. A young man (Mark Duplass), still inconsolable a year after his brother’s death, is urged by the latter’s girlfriend (Emily Blunt) to chill out at her family’s cabin, where he unexpectedly encounters her attractive sister (Rosemarie DeWitt). In some mumblecore movies the semi-improvised dialogue can be engulfed by hipster irony, but the acting here is so skilled, and the emotional terrain so rocky, that Shelton manages to break past the genre’s narrow social parameters to a moving story of grief, betrayal, and devotion.