The Exile Waiting

From the front flap of the Gollancz first printing: The time is the distant future and Earth has been rendered uninhabitable for much of the year by terrible storms during which its only city, Center, constructed around a natural cave system, is closed off from the outside. Mischa, a young thief whose capabilities are enhanced by hereditary mutation, is trying to escape with her drug-addicted brother from the dominance of her uncle. So when a starship arrives commanded by the twin alien pseudosibs, Subone and Subtwo, Mischa seizes her chance. She is soon assigned for training to Jan Hikaru, who quickly realises that Mischa has immense abilities. All goes well until Subone, in a moment of cruelty, lures Mischa's brother into a fight and then kills him. Mischa attacks Subone and both she and Jan have to flee the wrath of the brothers and take refuge n the deep cave system underneath the city.