A man struggling with his feelings following a death in the family throws himself into an even more uncomfortable situation in this independent comedy-drama from filmmaker Lynn Shelton. A year after the death of his brother, Jack (Mark Duplass) is still having a hard time dealing with his loss, and when he has an emotional outburst at a family get-together, Iris (Emily Blunt) steps in to offer some help. Iris is one of Jack's closest friends and was also dating his brother at the time of his death; Iris' family has a cottage on Puget Sound, and she suggests Jack bike up there and take some time to come to terms with his troubles. Jack accepts her offer, but when he arrives at the cabin, he discovers it's occupied by Iris' sister Hannah (Rosemarie DeWitt), who is in need of some alone time after breaking up with her lover. Jack and Hannah discuss their heartache over a bottle of tequila, and end up stumbling into bed. The next morning, Iris arrives, and Jack is desperate to keep her from learning he had sex with her sister; Hannah agrees not to tell, but as she bonds with Jack, she also senses his feelings for Iris aren't as platonic as he cares to admit. Your Sister's Sister received its world premiere at the 2011 Toronto International Film Festival.
by Mark Deming synopsis