The Ender Quintet #1: Ender's Game

In order to develop a secure defense against a hostile alien race's next attack, government agencies breed child geniuses and train them as soldiers. A brilliant young boy, Andrew Ender Wiggin lives with his kind but distant parents, his sadistic brother Peter, and the person he loves more than anyone else, his sister Valentine. Ender's skills make him a leader in school and respected in the Battle Room, where children play at mock battles in zero gravity. His psychological battles include loneliness, fear that he is becoming like the cruel brother he remembers, and fanning the flames of devotion to his beloved sister. Ender's two older siblings are every bit as unusual as he is, but in very different ways. Between the three of them lie the abilities to remake a world. If the world survives, that is.