• Your Sister's Sister

Maybe it was too much to ask that writer/director Lynn Shelton follow up a debut as bracing as Humpday with something just as brilliant. But it’s hard not to watch Your Sister’s Sister with a certain frustration that it feels built on so much flimsier a foundation.

As with Humpday, the premise seems to come from a high-concept studio comedy. Jack (Mark Duplass), still a mess a year after the death of his brother, is encouraged by his best gal-pal, Iris (Emily Blunt), to take some “me time” at her father’s cabin on a Puget Sound island. When he gets there, he unexpectedly finds Iris’ gay half-sister, Hannah (Rosemarie DeWitt), also looking for a getaway after the end of a long relationship. And one drunken night later, the complications are just beginning.

Shelton once again builds her story on scenes that are semi-improvised with a disarming fluidity and charm. Jack and Iris’ tequila-soaked late-night confessional session and the sisters’ slumber party are particularly terrific, building an authenticity into the character dynamics that brings the performances to life. By filling the first hour with so much smart, funny, insightful humanity, Your Sister’s Sister sets the bar fairly high.

Yet where Humpday perfectly combined that naturalism with its potentially farcical setup, Your Sister’s Sister lacks some crucial connective tissue. A character detail like the story of Iris and Hannah’s serially philandering father winds up feeling like a gimmick to explain Iris’ English accent, rather than a part of what defines the women’s relationship; Jack’s own relationship history feels like an important but never-explained black hole. The setup is so strong and so promising that the soggy resolution seems to belong to another, far less savvy filmmaker. As enjoyable as it is to spend time with these characters, they deserve better than “a-ha” moments tacked on to the end of a montage.

YOUR SISTER’S SISTER

Twitter: @ScottRenshaw