An improvisational three-act comedy featuring three appealing actors,” Your Sister’s Sister” is a study in triangulation set in a light-filled cabin in the San Juan Islands north of Seattle.

A slight but satisfying film ingeniously structured by writer/director Lynn Shelton (“Humpday”) and deftly improvised by Mark Duplass, Emily Blunt and Rosemarie Dewitt, it’s a triangle whose hypotenuse is forever shifting.

At a memorial for his brother, Jack (Duplass) offers a roast instead of a toast, much to the discomfort of others. Iris (Blunt), his brother’s ex, promptly stages an intervention, commanding Jack to her family cabin so he can clear his head.

He arrives to encounter Iris’ sister, Hannah (Dewitt), who has just decamped from a seven-year relationship. The first night it’s Jack, Hannah and a bottle of tequila. The two have sex. The following morning, Iris makes a surprise visit.

Awkward Jack implores Hannah to keep their close encounter a secret from Iris. Even more awkwardly, Iris confides a secret to Hannah. For her part, Hannah does not immediately share her secret. But from the expression on her sphinx-like face, we know she has one.

What ensues is surface civility with so many emotional undercurrents and crosscurrents coursing beneath that it’s a wonder the principals remain buoyed, if not always buoyant.

Their secrets, naturally, obstruct the intimacy each of the unmoored Seattleites craves. It is ruefully funny to watch, among the many avoidance conversations, discussions of whether gluten-free pancakes are superior to old-school flapjacks. And even funnier to watch first Jack, than Iris, pull Hannah out for sidebar conversations.

Before its blackout finale the geometry of “Your Sister's Sister” recedes and the film assumes the shape of a high-stakes poker game. The bemused players wonder, does the love between sisters trump that between romantic partners? Does love trump friendship?