Wild Magic by Cat Weatherill.

Dark, magical and exciting, Cat Weatherill's new fantasy takes the legend of the Pied Piper and gives it an unexpected twist. Mari and her brother were special. Their mother was a magical elf. She lived in another world where the Pied piper lived. A world full of danger, wild magic and adventure.

But she died sadly leaving the children and their weak farther remembering her beautiful, kind life. Mari and her brother one day followed the Pied Piper with the other children to that world. The Piper was very mysterious, he wanted something from the chidren, something special, powerful that could break an old-century curse. While the children were being transformed to animals, the brother was way behind. He was actually the chosen one, the kid the Piper needed. He had the power of his mother too.

He was going to save his sister from the Beast there. Which was the Piper. Breaking the curse wasn't easy, it is cruel and dangerouse for the others. The rest your have to fead it. It was the BEST book i ever read. It si in the library i think...

You will have to find out in the end.