Acorna's children - Anne McCaffrey

Khorii, the rebellious daughter of the near-mythic Acorna and her lifemate, Aari, has followed in Acorna's footsteps leading their people from danger, but the pressure to succeed and fulfill a legacy is tremendous. For the deadly foe that has ravaged the known worlds and weakened even her famous parents has launched its final assault, and only Khorii and her newly discovered sister, Ariin, are able to stop the brutal attack. But success is elusive, and fragile, and even time itself may not be enough to help their desperate quest to save their family...much less the universe.