Jim Henson's The Storyteller: Shapeshifters #1

Dain, a nefarious, power-hungry magic user and Lir's brother-in-law, plots to steal the throne from Lir and his family by turning Lir's children into swans and ascending the throne himself. Fionna, the oldest, wants only to gain revenge against Dain, in order to continue her quest to help her father secure his fierce and fearsome legacy as a renowned warrior. While Fionna attempts to find a way to break their curse, the swan-children are forced to wander near and far for nearly a thousand years.
They endure much hardship and sorrow, but face the curse together...finding solace and strength in one another.