Amber Castle 
Spell Sisters: Grace the Sea Sister [EPUB ebook] 

When eleven-year-old Gwen ventures into the forest beyond her castle home she comes across the magical island of Avalon and her life changes forever. The lady of the lake, Nineve, asks Gwen to embark on a quest to protect the enchanted island of Avalon from the evil sorceress Morgana la Fay.
Morgana has imprisoned the eight Spell Sisters of Avalon throughout the kingdom and stolen their magical powers. 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.
In their fifth adventure Gwen and Flora face stormy seas and dangerous cliff-climbs to save Grace the Sea Sister from Morgana”s spell.