On the seas to Troy - Caroline B Cooney

When she is six, red-haired Anaxandra is taken from her island home in the Aegean Sea by King Nicander of Siphnos. She is to be a companion to his daughter, Princess Callisto and life for her starts over. Six years later, fate intervenes again. Siphnos is sacked by murderous pirates, and Anaxandra is the only survivor. When a fleet of ships stops on the island to investigate, she pretends to be Princess Callisto in order to survive.

The ships belong to Menelaus, king of Sparta, who takes her back to his palace. But his beautiful, dangerous wife Helen does not believe that this flame-haired girl is Callisto. Anaxandra manages to avoid Helen and stay safe - until two Trojan princes, Aeneas and Paris arrive in Sparta. But when Menelaus is away, Paris and Helen begin a passionate affair that preludes war between Troy and Sparta. When they leave for Troy, Anaxandra follows and is soon caught up in both the fighting and a struggle with herself over where her loyalties lie.