Happiness Falls by Angie Kim

 Mia and John are 20-something twins and the children of a Korean mother and American father. They have a younger brother named Eugene who has severe autism and a rare condition called Angelman Syndrome. Eugene cannot speak. This turns out to be a big problem when Dad and Eugene go out for a walk in the woods and only Eugene returns--in quite a state. But he can't tell anyone where Dad is. The family is sure he will turn up, but when he doesn't they call the cops. The female cop who comes to the house acts very friendly but really thinks Eugene did something to the father. So they stop trusting her and start their own investigation which leads to a lot of questions. Was the father having an affair? Is he lying injured in the woods? Did someone rob the father and Eugene out in the woods?

It's an intriguing story about how everyone treats the brother who can't communicate but holds all the answers. And how everyone's perceptions of each other change the longer the search for the father continues.