I like the concept of the book, but there are many places you could be a lot more descriptive in your writing. At some points I can't tell if this is a novel or a short story? If it's the former, try re-writing the chapters to be more descriptive and longer. The one thing to keep in mind, of course, is "Show, not tell." Don't just say "She was anxious to be visiting her family," talk about how she paced in the bathroom for hours, or the edges of her nails were red and ragged.
Read the story now
Another thing is that most parts of the book aren't as accurate as you would hope from a true crime book, if that's what you wanted to write. And I realize that yes, this is fiction, and fiction means some things are wrong, but the inaccurate parts are just plain confusing. If, like you say, John has the mindset of a four-year-old (which isn't medically possible), he would likely have a genetic disorder and not brain trauma from an accident. After his death, it's not likely the police would run an autopsy unless they had some sort of suspicion of any "dirty action", like murder, and as Gemma was visiting family at the time of his death, she had an alibi and any tip-off from Lisa would be met with derision. The time after his death gets a little blurry, because Rosa gets married and has a kid, but you say Gemma was only in her late thirties at the most. What are you trying to tell with this story? I like it, it just needs a little work and a lot more research.