Amelia the Silver Sister
Amber Castle Audio Book

The evil Morgana Le Fay has spirited the Spell Sisters of Avalon away from the magical island and trapped them throughout the kingdom. It's up to Gwen, her best friend Flora and a very special horse named Moonlight to find the sisters and return them to Avalon before its magic is lost forever. Amelia the Silver Sister is imprisoned in an old castle and Gwen and Flora are determined to rescue her. But when Morgana attacks the castle with a hurricane, the girls become trapped too! Can they escape and return Amelia to Avalon?