Sevenwaters, book 4: Heir to SevenwatersJuliet Marillier2008

The chieftains of Sevenwaters have long been custodians of a vast and mysterious forest. Human and Otherworld dwellers have existed there side by side, sharing a wary trust. Until the spring when Lady Aisling of Sevenwaters finds herself expecting another child - a new heir to Sevenwaters.

Then the family's joy turns to despair when the baby is taken from his room and something...unnatural is left in his place. To reclaim her newborn brother, Clodagh must enter the shadowy Otherworld and confront the powerful prince who rules there.