A ski-lift gondola, rumbling up the side of a Swiss mountain, is the means by which 12-year-old Simon (Kacey Mottet Klein) gets to work.

His job: picking pockets, stealing boots and backpacks, taking money and even sandwiches from vacationers, bringing his bounty back to the apartment in a public housing high-rise that he shares with his adult sister (Lea Seydoux). That’s how they get by, he and Louise: He’ll hawk his wares, pocket the cash, share the food. As for Louise, she’ll go off on dates with guys who pull up in cars, returning late in the night, or the next day.

“Sister,” Ursula Meier’s dark fairy tale of a film, is beautifully observed, with a riveting and unstudied performance from Klein. It tracks its little thief as he prowls for loot — and crosses paths with strangers who hold out the possibility of friendship. There’s a British cook (Martin Compston) working in one of the ski resort’s restaurants, who reluctantly aids and abets Simon, and there is a well-to-do mother (a memorable turn from Gillian Anderson), there with her children, enjoying the snow and sun. Simon insinuates himself into her world.

Simon and Louise are like lost children from some old fable. They share a sense of isolation, an unspoken acknowledgment that life is moving apart from them. And they share something else unspoken as well.

It’s haunting and sad. And absolutely worth seeing. (At the Tivoli.)