Changer's Blood (Book 2 of the Balancer's Soul cycle) - The Balancer's Cycle, #2

After Connor blended his soul with Sarah something strange and wounderous happened, their powers are unknown and seem to grow by the day. Even Sarah's ancient parents haven't a clue as to why their abilities are so unique and unprecidented. To make matters worse both Connor and Sarah have unfortunately revealed their powers over the elements to an outsider, his abnoxious little sister Kara, but they werent the only one to have an enormous secret.
Mark, Kara's boyfriend revealed himself to be more than human himself, but unlike her brother and Sarah who are Balancer's he is a Changer who can manipulate his very body at will. With Connor and Sarah becoming more powerful and learning just what they can do, Kara has to decide how to cope with immortals, supernatural powers and her own mortality. Family is such a welcome pain.