Escape From Olshek's Castle - The Forgotten Portal, #1

Allie McGee, straight-A student and rule-follower to the core, comes home from school one day to find that her little brother Pete is gone. She remembers the imaginative stories he told about a magical tree in the woods behind her house and she sets out immediately to find him. She begins to realize his stories were true when she finds the tree and is transported to a different world, only to discover that Pete's been kidnapped by the wicked King Olshek.
She must go on a dangerous journey to get to the castle where her brother is being held captive. Will Allie dare defy Olshek? Or will she and Pete be trapped in his castle forever?