I read this series about 15 years ago and so my memory is spotty, but it's been bugging me lately. The main characters are a pair of royal siblings (possibly twins?). The girl is raised in the palace, but the boy is kidnapped as an infant and replaced with a golem. As a result the royal family believes that their son is mentally handicapped because he is very slow both mentally and physically.
The real prince is raised by magical creatures, possibly fairies, although I can't remember exactly except that I'm almost certain that the adoptive parents are very tiny. The only other thing that sticks out to me is one of the main types of magical creatures are centaurs where the animal part isn't a horse. I distinctly remember there being a character with the upper body of a person but the main body of a cat.