Promises (Syrenka, #1)

Sometimes following your heart can end up destroying the ones you love. When seventeen-year-old Eviana Dumahl is faced with the responsibility of an arranged marriage and clan leadership, she is forced to choose between the life required of a mermaid and one of a teenage girl simply infatuated with the wrong guy. Kain, her devoted fiance, would make a wonderful husband except that she's been in love with Brendan, a shape-shifting selkie, ever since they were kids. Choosing to abandon her family, her clan, and her life with Kain will have dire consequences far beyond anything she could have imagined. A war is brewing amongst the clans and Eviana unwittingly becomes a pawn in the intricate schemes of a twisted mastermind. With Brendan's life on the line, she has no choice but to involve those she once considered friends. Amidst encountering senseless tragedies and immense loss, Eviana discovers that she is more valuable as a clan leader than anyone ever suspected. Her survival is not only dependent upon the loyalty of her friends, but also on her acceptance of a life she so adamantly tried to escape. Promises is the first book of The Syrenka Series trilogy following Eviana and her friends through the perils of growing up in a discrete world that inherently threatens human society."