Your Sister's Sister - Emily Blunt and Mark Duplass
Your Sister's Sister - Iris (Emily Blunt) is going to need that drink when she finds out what her best friend Jack (Mark Duplass) has been up to with her sister © 2012 IFC Films

Your Sister’s Sister is the second Emily Blunt movie in a row that saddles the always enjoyable actress with a badly miscast soulmate. The secret longing that Blunt’s character Iris feels for doofus man-child Jack (Mark Duplass) is as off-puttingly implausible as her pairing with Jason Segel in April’s The Five-Year Engagement. That’s a shame, because Blunt manages to be genuinely likable even in this talky, unlikely and frustrating bore.

Jack is the movie’s main character, an unemployed overgrown schlub whose dead brother was Iris’ ex-boyfriend. Jack is still obnoxiously upset about his brother’s death a year later. He and Iris remain such good friends that she suggests he spend some time alone at her father’s isolated island cabin because he needs some “head space.”

Jack arrives to find Iris’ sister Hannah (Rosemarie DeWitt) already staying there. She’s getting over a breakup with a longtime female lover, but she and Jack bond during an interminable conversation and a musical montage’s worth of tequila shots. They end up having sex, which strangely is only the second most offensive-to-lesbians aspect of the film.

Your Sister's Sister - Emily Blunt and Rosemarie DeWitt
Your Sister’s Sister – Iris (Emily Blunt) and Hannah (Rosemarie DeWitt) share some sisterly pillow talk © 2012 IFC Films

Jack and Hannah agree to keep what happened the night before just between them when Iris shows up unexpectedly the next morning. That means most of the rest of Your Sister’s Sister involves waiting for their secret to hit the fan when Iris realizes that the man she loves has hooked up with her sibling.

Acting more like high-schoolers than adults, Jack and Hannah misdirect and lie to keep Iris in the dark about what they did. A thin layer of melancholy doesn’t keep this simpleminded plot device from seeming any less like a teen-comedy cliché.

A bigger problem with the largely ad-libbed screenplay (by director Lynn Shelton) is its tiresome dialog. The actors improvised many scenes with alternately awkward and showy self-consciousness. The dialog isn’t Kevin-Smith bad, but conversations suffer from the same kind of desperately unnatural chattiness that makes Smith’s movies so excruciating.

Jack is supposed to be charming enough to intrigue both Iris and Hannah, but Duplass plays the character like an immature and vaguely creepy slob. The more you see him, the more you want to see less of him. The guy also is no Zac Efron in the looks department, which would have helped explain his appeal on at least some level.

DeWitt gives Hannah just a hint of mental instability, which fits the role. But Hannah’s function in the story ends up being irredeemably contrived.

Only Blunt manages to rise above this weak material. Iris’ whispered pillow-talk scene with Hannah seems forced, but Blunt delivers it with enough sincerity and heart that it almost works anyway. She also has one of the most convincingly endearing laughs in show biz.

When it finally comes, the movie’s big reveal is mishandled by occurring offscreen. Also, an unsatisfying final-act twist has no resolution whatsoever, which feels like a complete cheat. Consider yourself warned.

[Rating: 1 star]

Your Sister’s Sister US and Canada theatrical release Friday June 15, 2012. UK release June 29, 2012. Australia release September 6, 2012

Your Sister’s Sister Official Facebook page.

James Dawson

Jim is Film Review Online's Los Angeles based reviewer.