Name of a movie about a girl who doesn't talk and mom builds stairs in the woods?

In this movie, a young girl is treated for mutism, (perhaps post-traumatically induced) after (i think) her dad dies, although a lot of her behaviors suggest autism. Her mom and brother try to "unlock" her from her inner world by building a huge staircase hanging from the trees in the woods near their house. This movie was on HBO or STARS about 4-6 years ago or so. Any information on main actors,etc would be great, but the name of the movie would be best!