As an only child raising an only child of my own, a lot of sibling stories tend to fall flat for me/us, but I found this one really engaging and relevant beyond just the obvious brother/sister conflict. It explores an idea that any child (or adult) can relate to - what would happen if our most sinister/selfish desires were actually granted? Although, trust me when I say, it's really a very light hearted exploration of that idea. I really like Matt Phelan's accompanying illustrations which reminded me very much of those in In a Blue Room, but perhaps even nicer? I'd definitely like to see more from him. I found myself wishing that the mother in this story wasn't such a distracted mega-grump, but it does serve a purpose in the story.
For another (much more intense!) book along similar lines I recommend Outside Over There