The Mermaid's Sister (Water Horse #2)

Mia is cousin to Olivia, the duchess of Venice; a ruler loved by her subjects for her command of magic and for saving the city from the flood. But Mia harbors a secret hatred of Olivia - her brother and mother were banished by the princess years before, and Mia has never forgiven her. Determined to ignore the powerful magic that is part of Olivia's reign, Mia finds herself lonely and isolated - until the appearance of Ara, a mergirl who may just hold the key to Venice's victory against the deadly Talish invaders...