I watched this recently, and strongly recommend getting a hold of the DVD to watch if you can.
I'm really glad I didn't go see this at the cinema as I spent large amounts of the movie in tears.
For me it was a window into what Annie's life will be like as a sibling of a child with autism. There is the downsides, but they are far outweighed by the positives. I think Annie is a luck girl to grow up with a sister who can show her the world from a whole new point of view.