HighTower Fairytales: The Little Mermaid Series by Jacque Stevens

I always knew this day would come. That just as my hair lengthened and my curves formed, I would make the transition from girl to woman. To siren. Arianna is the youngest daughter of a siren. To become a siren herself, she must kill a man and sacrifice his heart to the God of the Deep. But she never expected to be assigned to kill a prince or to fall in love with him. Torn between her family and her new love, Ari must decide what lies in the depths of her soul.