Fire in the Star: The Unicorn Quest 3

Claire Martinson has always depended on her older sister, Sophie, to take care of her, but now its Claires turn to protect her sister. On the trail of the last unicorn, Claire sets off on her own to make things right in Arden - the magical land she and her sister discovered when they climbed their great-aunts chimney. But there are terrible forces working against her.
Queen Estelle has returned, and she will not stop until she has taken revenge on all those who have opposed her. Claire, Sophie, and their friends must convince Ardens four guilds of magic to set aside the past if they are to have any hope of a future. Because if the queen has her way, Claire and Sophie will never return home and the unicorns of Arden will be lost ... forever.