Return to Aqualor by Debra A. White

In the Arvak Kingdom, the terrible tyrant Chasqua has escaped confinement. With the portal to the human world sealed, the Arvak people don't know if they can cope without the help of their Paderion, the human boy Drew Barret. Since returning from their last adventure in Aqualor, Drew and his younger sister Alexia have been set on finding another portal to visit their friends. They soon find themselves pulled to the magical world and confronted by Merfolk who desperately need their help to restore peace and freedom to the realm. Embarking on a journey filled with tests of both mind and body, Drew and Alexia's are pushed to their limits as they reunite with old friends, meet new allies, and confront terrifying dangers. The siblings must rely on each other to fight for the future of Aqualor. Drew knows that sometimes sacrifices must be made for the greater good – and he will stop at nothing to protect those he loves.