There are some films which are spoiler proof. That is, they are so bizarre or indescribable that it's virtually impossible for someone to be told enough about the movie to ruin the experience of watching it. On the other hand, there are films whose plot is so basic and familiar that you could just about write the movie from a one-sentence description of it. Your Sister's Sister is firmly in the second category.
Jack (Mark Duplass) is having a rough time. His brother Tom died a year ago, and he's been in a real funk since then. His best friend Iris (Emily Blunt) has finally had enough of it and suggests that he get away to her family's lake cabin to take some alone time and clear his head, and he reluctantly agrees. But when he arrives, he's surprised to find the cabin occupied. Iris' half sister Hannah (Rosemarie DeWitt) is doing her own bit of wallowing after ending a seven-year relationship.
Once they get past their awkward meeting, they separate to rooms in opposite corners to suffer alone. But neither can sleep, and they each make their way to the kitchen for what turns into a late-night drinking binge. One thing leads to another, and drinking turns into sex. It's something they would have surely regretted on their own the next morning, but the process is accelerated when they hear Iris pulling into the driveway to surprise Jack. Jack and Hannah hurriedly agree that it would be a very bad idea to let Iris in on what happened, adding yet another layer of awkwardness to their time together.
Your Sister's Sister is a surprisingly dull and poorly-executed film. I like all three of the leads, and while their acting isn't particularly inspired, it is also far from the biggest problem with the movie. That honor goes to the film's audio, which is almost painful at times. Most of the movie has no score and sparse dialogue, so the only thing you hear (aside from audience members shifting in their seats) is a hiss coming from the speakers. And when someone does say something, it's often loud and harsh-sounding which distracts you from what they have to say. When there is a score, it's basic and unremarkable.
I was also unimpressed with the visual quality, with the digital image looking very grainy, especially in dark scenes. The scene in which Jack arrives at the house is particularly bad, and the light level is so inconsistent that it almost appears to be pulsing. The extensive use of a handheld camera with no stabilization means that the frame is constantly bobbing up and down and side to side. It's not severe enough to make you seasick, but it's definitely noticeable.
The audiovisual problems would be a problem for just about any kind of film, but they're almost inescapable in this film because it's so predictable and boring that you're compelled to take note of what's happening outside the story. There are really only two surprises in the entire film, but the first doesn't do much to help improve it, and the second actually works more to its detriment.
I hadn't expected a lot of originality from the movie, but I'd hoped that the quality cast would be given a smart enough dialogue to win me over. But the film's complete lack of creativity, especially when accompanied by its technical failings, destroyed any chance I might have had to enjoy it.