It's comforting to watching dramatic and comic fiction regarding the things some of us actually have to deal with in life. When done right, those kinds of films are worth your time. YOUR SISTER'S SISTER happens to be one of them.
Don't be surprised if you find yourself nodding off mid-movie, bored silly by a trio of good actors stuck in a dull scenario about a young man caught in a highly implausible kerfuffle involving two sisters and a leaky condom.
Your Sister's Sister is a fine piece of character observation. It's low-key, casual, and tremendously insightful. It won't be for everybody, but that's a good thing. It's a strong, focused piece of work.