Simple and slight but affecting in its own way, "Sister" showcases Agnès Godard's picture-perfect cinematography and a pair of fine performances from newcomer Kasey Mottet Klein and the stunning Léa Seydoux (2011's "Mission: Impossible – Ghost Protocol"). The two co-stars play 12-year-old Simon and his elder sister Louise, parentless and scraping by in a low-rent tower apartment that sits at the foot of a ritzy Swiss ski resort. As Louise goes from one dead-end job and no-good boyfriend to another, Simon proves more profitable, posing as a guest of the chalet and pilfering nice skis and equipment for money. The two of them are scraping by, but going nowhere, and, as is eventually revealed, the strain on Louise to care for Simon goes much deeper than that of a young woman struggling—and so far, failing—to be the adult that her brother needs her to be.

If the landscape at the bottom of the mountains is bleak and unsparing, it is a different world up at the ski resort, a figurative castle in the clouds. Up there, Simon can be a different person, befriending a tourist, Kristin (Gillian Anderson), with two children while going by a false name, Julien. These charades can only carry him for so long, however, the truth lying in the place where the sky trams deliver him at the end of the day. As Simon, young Kasey Mottet Klein is a self-assured natural; yes, he steals things that don't belong to him and doesn't always do the right thing, but he is an adrift pre-adolescence without the guidance he needs. If anything, he takes better care of Louise than she does of him, but that doesn't make the shift in roles right. As Louise, Léa Seydoux is excellent, her character in need of growing up herself since she never got a chance to all those years ago.

The relationship at the center of "Sister" is, in essence, the entire movie, or at least the heart of it. They are contentious with each other, but also protective when need be, and the reveals that occur midway through that bring clarity to their past are surprising but wholly plausible, a lucid extension of the narrative. The film, more a slice-of-life than a formulaic picture hinging on plot, can be nearly too low-key at times, stretching to fill out an hour an a half. This forces one to concentrate on Simon and Louise, then, and the very ending, saying so very much in a breathtaking final shot, is the flawless capper to a movie that otherwise is anything but. Unassumingly but surely, "Sister" leaves a poignant impression for viewers patient enough to let it take hold.