The three emotionally adrift characters who share 90 percent of the screen time in “Your Sister’s Sister” could be your best friends, your siblings, your sons and daughters, or they could be you.

This wise, funny and uncomfortable new film, opening in area theaters today, begins with a scene that sets the tone for the rest of the film. At the one-year commemoration of his brother Tom’s death, brooding Jack (Mark Duplass) leans against the back wall with a beer while a friend waxes nostalgically about Tom’s selfless humanitarianism, painting the picture of a bona fide angel. When Jack finally speaks, he punctures the speaker’s hagiography, cutting through the B.S. to state that Tom, in fact, was a bully in his school days, and that he became a do-gooder to score chicks.

“Your Sister’s Sister” is not about a man dealing with his dead brother’s baggage, but this disquieting overture to a complex love triangle lays the thematic framework that will flourish into a perfect morality tale. In this case, Jack insists on honesty even when whitewashing or concealment is more convenient – an ethos that will be put to the test during the naturalistic dramedy that follows.

Most of the film takes place on a private island in the Pacific Northwest owned by the father of Jack’s best friend Iris (Emily Blunt). Iris arranges for Jack to take a sabbatical at the family’s cottage there – a few days of off-the-grid freedom with no Internet, no TV and lots of verdant flora and blue waters. What she doesn’t know is that her sister Hannah (Rosemarie DeWitt), a lesbian who has just ended a seven-year relationship, has also taken up lodging in the remote dwelling, leading to a drunken decision that will impact all three of them in life-changing ways.

The writer-director of “Your Sister’s Sister” is Lynn Shelton, the mumblecore auteur who gave us “Humpday” in 2009. In that film, two straight guys agree to have sex with an each other for an avant-garde porn film; likewise, in “Your Sister’s Sister” a character momentarily flips her sexual orientation, but with a twist that justifies and deepens the movie.

Shelton has an incredibly accurate ear for young peoples’ cadence and diction, so it’s no surprise to learn that much of the film’s witty dialogue was improvised over a shoot lasting just 12 days. The action is de-dramatized and free of ostentation, with an unobtrusive score and a maverick ending that lets us decide the characters’ future – just some of the ways Shelton liberates viewers from the shackles of formula.

If the film’s style is fresh, its dramatic potency is timeless, harkening at least as far back to the moral fables of Eric Rohmer. It’s a cautionary tale about the dangers of secrecy and case study in the necessity of truth and honesty. When these themes are articulated this well, it’s almost a revelation.

“Your Sister’s Sister” opens today at Regal Shadowood in Boca Raton, Regal Delray Beach 18 and the Coral Gables Art Cinema. It opens June 29 at Living Room Theaters at FAU.