The Crown Of Fire - Book 1 of The Stormrider Chronicles

When Alexander Hayes and his sister, Octavia wake up in a strange cave, he thinks that he'd had a terrible dream, but upon further realization, he realizes that he's in another world; Ashiver. Hoping to replace their way home, they journey through the lands, blissfully unaware of how unsafe they are. When Aria Stormrider comes across two otherworlders being attacked by their oppressive ruler's followers, she comes to their aid. Unaware that by saving them, she is freeing her people. But beware, Ashiver is in a shell of its former glory. No longer do the trees sing, and the animals dance. For when the mage took over, the frost turned their joy and happiness into fear and darkness.