Mermaid Curse: The Black Pearl - Cooper, Louise

Since her family moved to Cornwall, Lizzy's world has been turned upside down by the discovery that she has a long-lost brother, Kes, and even more amazingly that their mother is a mermaid. Now Lizzy and her family must battle the evil sea queen and her servant, for Lizzy has a powerful treasure and the sea queen will stop at nothing to get it.