The Exile Waiting Paperback Vonda N. McIntyre

The time is the distant future. Earth has been rendered uninhabitable by terrible storms during which its only city, Center, constructed around a natural cave system, is sealed 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. When a starship arrives, commanded by the twin alien pseudo-sibs, Subone & Subtwo, Mischa seizes her chance. All goes well until Subone, in a moment of cruelty, lures Mischa's brother into a fight, killing him. Mischa exacts her revenge, but then has to flee the wrath of the aliens and take refuge back in the deep caves underneath the city.