Ice wolves / by Amie Kaufman.

Everyone in Vallen knows that ice wolves and scorch dragons are sworn enemies who live deeply separate lives. So when twelve-year-old orphan Anders takes one elemental form and his twin sister, Rayna, takes another, he wonders whether they are even related. Still, whether or not they?re family, Rayna is Anders?s only true friend. She?s nothing like the brutal, cruel dragons who claimed her as one of their own and stole her away. In order to rescue her, Anders must enlist at the foreboding Ulfar Academy, a school for young wolves that values loyalty to the pack above all else. But for Anders, loyalty is more complicated than obedience, and friendship is the most powerful shapeshifting force of all -- Goodreads.com.