I was really looking forward to Lynn Shelton’s film, which has been rapturously received at festivals worldwide. If I am honest though, I found it a bit underwhelming. Along with her previous feature, Humpday (2009), this could play as a double-bill under the heading ‘bizarre love triangle’.

That film’s unique premise saw a trio caught in emotional and sexual flux when two straight best friends resolve to make a gay porn film together. In Your Sister’s Sister, thorny familial and best-friend relations are complicated following an unexpected coupling. Just as Humpday brought an interesting dynamic to the table only to flounder mid-way, Shelton again squanders an early, spiky premise in a film that dissolves a bit too easily into triteness and cliché.

Jack (Mark Duplass from Humpday) is given tough-love advice from best friend Iris (Emily Blunt, luminous as always) after an alcohol-fuelled breakdown while commemorating the death of his brother (and Iris’ husband). She orders him to spend the weekend at her family’s remote lodge to sort himself out. When he arrives he finds Iris’ lesbian sister Hannah (Rosemarie DeWitt) – having fled a disintegrating relationship of her own. They hook up – a disastrous, drunken (and embarrassingly quick) fumble. When they awake, Iris has arrived at the lodge. Complications – naturally – ensue.

The first half of Your Sister’s Sister is far and away the strongest. In the opening sequence, Jack delivers an excruciating ‘tribute’ to his brother, revealing his own emotional scars in the process. A drunken tequila-fuelled conversation between Jack and Iris has believability and a witty charge. But the intimacy and truthfulness of these early exchanges tanks badly after a preposterous confession from Hannah derails the entire film.

What follows is equally excruciating – only in a different way. Disregarding her character’s histories (and logic), Shelton frantically begins tidying up any ambiguity or complexity. The script runs out of places to go and there are wordless, slightly twee montages set to folksy music with cuddles, laughter and tears before an unconvincing coda wraps things up.

There are real strengths here – understandably given the talent involved. The film is visually richer than Shelton’s previous work. The key difference is the quality of the performances and a pre-shoot improvisation process that has really paid dividends. The familial bond between DeWitt (a terrific actress so good in Jonathan Demme’s toe-curling Rachel Getting Married – the gold standard for recent affected, middle-class dramady) is superb and Blunt continues her run of fine work in OK movies. Someone write a really decent part for this woman, she’s effortless.

Shelton evidently has talent but it would be great to see her working with a stronger writer. The mumblecore mafia may feel they are flying in the face of Hollywood’s ‘tired’ formula with improvised scripts and loose structures, but too often their work feels every bit as calculated as an Aniston rom-com – just aimed at a different and significantly more self-satisfied audience. A polite and rather cosy méh-nages a trois that could’ve been sharper and funnier – but the sisters really do shine.