Mermaid Curse Series by Louise Cooper

Lizzy Baxter is thrilled when her family moves to Cornwall – she's always loved the beach. Lizzy doesn't realise, but she has a special connection with the sea – and the mysterical mermaids and silver dolphins who swim there. But there's much more to the beautiful blue-green world than magic. The evil sea queen is plotting to steal something precious – something that belongs to Lizzy. \n\nThe Mermaid Curse quartet is a perfect blend of fantasy and reality for readers aged 8–10.

Lizzy has always know she was adopted. But since her family moved to Cornwall her world has turned upside down - and a beautiful shade of blue-green. By chance, she discovered Kes, a brother she never knew existed, and the incredible truth about her birth mother...Now Lizzy and her new family must battle the evil mermaid Queen and her terrifying servant. For Lizzy has a powerful treasure and the Queen will stop at nothing to get it.

Taran, the evil mermaid Queen, is determined to get her hands on the silver pearl that will make her more powerful than ever. only Lizzy, Kes and their friends from the undersea world can stop her.But when Taran takes a very special hostage, Lizzy and Kes are faced with a terrible choice. Must they give up the pearl? Or do they have the courage to challenge Taran, and defeat her?

With Tullor safely behind glass at the Sealife Centre, and no news of evil mermaid Queen Taran, Lizzy almost believes that her troubles are over. Only, she can't help thinking that it's not like Taran to give up.When her sister, Rose, starts behaving very strangely, Lizzy wonders whether Taran might be up to her old tricks. Before Long, it's a race against time as Taran's final, terrifying plot to gain total power infolds.