The first book of The Heart Series is Wellspring. It follows the story of Serafina Wickham, who wants nothing more than to live as an Elemental should: wild, free, and at peace. But the most powerful of her race, an Earth Elemental named Delziel, wants vengeance against the Air Elementals that killed his family, even if that means enslaving the rest of his kind to get it. He’ll go to great lengths to use Sera as a weapon against his enemies, even take her twin sister, Eva, away.
Sera and Eva share everything; experiences, thoughts, feelings, all through a telepathic bond. They’ll die before they let Delziel separate them, but to stay together, they will have to either fight back or succumb to the will of a mad man.