A Mermaid's Swim Away

Estelle wants more than anything to leave her home, she's seventeen and ready to explore the world.
With the help of her stepsister, Rosella, Estelle is able to find her way to the land of the humans, the crystal shores. In exchange for Rosellas help, Estelle has to bring a man back for Rosella to love.
Along the process, what if Estelle falls for the man she was going to bring to her sister, what would happen if she broke her promise?