For long stretches in its first two acts, Your Sister's Sister is distinguished by a disarming sense of freedom and spontaneity—the feeling that we aren't so much seeing actors skillfully act out a script as we are watching real people interacting with each other, the camera merely there to capture the details of those interactions.
Witness, for instance, the first scene in which we see Jack (Mark Duplass) and Iris (Emily Blunt) together by themselves, after Jack has interrupted a friend's eulogy for his dead brother with his own less-than-glowing tribute. There's such a startling ease to Jack and Iris's interactions that one senses their years of friendship. Are these two people just friends, or is there unspoken romantic tension in some of the pregnant pauses in their conversation? There's a sense, seeing that Iris is willing to go so far as to arrange for a week-long getaway so Jack can figure his life out, that there may be something deeper going on than both of them are willing to acknowledge to each other, much less themselves.
A different emotional temperature, however, animates Jack's first encounter with Iris's lesbian sister, Hannah (Rosemarie DeWitt), an unexpected houseguest in the cabin Iris has arranged for Jack to stay in during his weeklong sabbatical. But even as Jack and Hannah make occasionally awkward attempts to try to get to know each other, it's possible to see much of Iris in Hannah, if nothing else, in the self-confident way they both carry themselves, especially around Jack. (Perhaps that's why Jack ends up having drunken sex with Hannah that first night in the cabin—a plot turn that ends up having enormous consequences when Iris surprises them both by showing up at the cabin the next day.)
Writer-director Lynn Shelton captures all of this in lengthy scenes in which she evinces a striking alertness to the many details of speech and body language that reveal multitudes about these characters, especially as they interact with each other. In a film like Your Sister's Sister, the things that are left unsaid between these people are just as important as the things that are openly stated. This is the kind of thing that John Cassavetes regularly excelled at in his films—and, like Cassavetes, especially early on in his career, Shelton relies heavily on improvisation to achieve this feeling of life happening right before our very eyes. The effect is often thrilling: Though Shelton's "script" may have dictated certain narrative and emotional beats, one can never quite predict how the actors will get to those points.
Your Sister's Sister is so amiably loose, so warmly observant in its depiction of character interactions, and so beautifully acted by its three principals that it's a shame the film eventually threatens to fall apart under the weight of an excess of plot twists and hedged bets. Up until its third act, Shelton's film is an absorbing look at characters handling, in their own sometimes problematic ways, both various forms of grief and loss (Jack still dealing with the death of his brother, Hannah dealing with the recent end of a seven-year relationship) and unspoken desires (Hannah's desire to be a mother, Iris's secret love for Jack, Jack's own desire for a sense of normalcy in his life). But then Shelton piles on complication after complication, trying to drum up drama among these three people, and much of it can't help but feel a bit too artificially manufactured after the striking naturalism of what came before. Worse, the film's third act turns what promised to be a fairly provocative film in regard to its take on (straight and gay) interpersonal friendships and romantic relationships into something disappointingly safe and conventional. Like Shelton's last film, Humpday, which used a gay-porn conceit to poke fun at male egotism, Your Sister's Sister ends up sticking the landing, preferring tidiness when the rest of the film suggested something messier and potentially more transgressive in the works.