We Do Not Welcome Our Ten-Year-Old Overlord By Garth Nix

It’s not fair. Kim’s always lived in the shadow of his younger sister, Eila – which is not how it’s supposed to be. Eila is a prodigy, and everyone talks about how smart she is but, in Kim’s eyes, she has no common sense. One day Kim and Eila are walking in the woods, and Eila finds an enigmatic, otherworldly object. Kim thinks it’s bad news – but Eila begins to commune with it. Kim has every reason to be worried, because soon Eila is able to control the minds of everyone around her – in ways she says is the best for everyone. Kim needs to put his problem-solving mind to the ultimate test. How can he save his sister, and also save the world from the thing his sister has become and the forces she has unleashed?