Movie, Sister is all all about Simon (Kacey Mottet Klein) who lives with his older sister (Lea Seydoux) in a housing complex below a luxury Swiss ski resort. While she the older one goes through different jobs and relationships the younger one who is twelve starts to take on the responsibility of providing for the two of them.
Each and every day, he takes the lift up to the ski world above, he starts stealing equipment from rich tourists to resell to the local kids down in the valley. He is at the point of keeping the two of the, afloat with his small-time hustles and his sister is so thankful for the money he brings in.
But, when Simon partners with a crooked British seasonal worker, he begins to lose his boundaries, affecting his relationship with his sister and plummeting him into dangerous territory.