This shouldn't be classified as a mystery. It's really not interested in interrogating the story as a mystery at all, and basically slams the door shut on that. It's a family drama, lit fic, and (as with Kim's Miracle Creek) a fictionalization of life as a family taking care of a nonverbal autistic son. There's no real resolution, so if an ending that doesn't have the answers is a no, steer clear.
If that's what you're up for though, I think it's pretty well written. The perspective being from one of the family's children, rather than one of the parents, was really interesting to me. I wavered the whole time between loving and hating the MC, which is a feeling I tend to enjoy while reading. She's 20, so she's technically an adult, and she's very smart, but she's so frustrating throughout.