My Gym Teacher Is an Alien Overlord
The second book in the My Brother is a Superhero series.
Zack and Lara have superpowers. Luke has new school shoes and a burning sense of resentment. He KNOWS that aliens disguised as gym teachers are about to attack Earth but will anyone listen? No.
So one dodgy pact with a self-styled supervillain later, and Luke is ready to save the world. He just needs to find his trainers.