Changelings (Christine Soontornvat)

1.

"All Izzy wants is for something interesting to happen in her sleepy little town. But her wish becomes all too real when a mysterious song floats through the woods and lures her little sister Hen into the forest...where she vanishes. A frantic search leads to a strange hole in the ground that Izzy enters. But on the other side, she discovers that the hole was not a hole, this place is not Earth, and Hen is not lost. She's been stolen away to the land...