?Your Sister?s Sister? Review Roundup: Lynn Shelton's Latest Is Winning Critics Over

The film, which screened at the Tribeca Film Festival and had its red carpet premiere in Los Angeles on Monday, stars Emily Blunt, Mark Duplass, and Rosemarie DeWitt in a peculiar, boundary-pushing love triangle. Iris (Blunt) sends her best friend Jack (Duplass) -- who is mourning the year-old death of his brother and Iris's ex -- to take a mental health break at her father's remote cabin; not realizing her lesbian half-sister Hannah (DeWitt) will also be there. When Iris arrives the next day, she has no idea that her sister and her BFF had a drunken sex romp the night before. Of course, Iris has a revelation that she might be falling for Jack, further complicating an already messy situation.

Like Shelton's previous feature, Humpday, the script is improvised, though not to the same degree. Humpday -- about two straight male friends who are dared to make a gay porno together -- was entirely improvised. Most of the scenes were written out but only somewhere between 20 and 30% of what you see on screen is as-written, Shelton told Time.

The film is largely getting good reviews so far -- it currently has a 92% approval rating on Rotten Tomatoes -- though some critics have had a hard time with the dialogue. The New York Observer's Rex Reed complained that Blunt is totally incomprehensible in one scene.

But it didn't entirely ruin the experience for the critic. Like all Lynn Shelton films, the characters develop big intimacies in small doses, with humor that swings from understated to raunchy, Reed wrote. It takes a while for them to break through the roadblocks of rhetoric and reach your heart.

Shelton's film more easily won over the Associated Press critic: Even when things get a little crazy and maybe even too soapy, 'Your Sister's Sister' always feels like it's rooted in a tangible reality, a place of unpredictability and abiding humanity.

A Time review highlights the movie's ambiguous ending: But no matter how poignantly undefined the specifics of their future are, Shelton has already shown them making a crucial decision together.

The film marks a return to Dewitt in a supporting role as a sister. She previously played the more together sister to Anne Hathaway's basket case in Rachel Getting Married. In the United States of Tara, DeWitt played the exasperated but ultimately supportive sister of Toni Collette's character, who has multiple personality disorder. She also appeared on a few episodes of Mad Men as Midge Daniels, a struggling artist and former flame of Don Draper.

Your Sister's Sister marks another actor/director collaboration between Shelton and Duplass, who starred in Humpday. Shelton praised Duplass' improvisation skills.

Mark is such a veteran, a really comfortable improviser, she told NBC. He was like the ringleader when he was in the scenes with the gals, would sort of lead them. They would follow wherever he went, and it was just fantastic to have that kind of confidence.

Your Sister's Sister opens in limited release on June 15, offering a respite for indie film lovers buried under summer blockbusters. The film will head to theaters in the UK on June 29.