The Mermaid’s Sister by Carrie Anne Noble

I used to listen to this on audiobook as a little kid (even though it is undoubtedly a bit intense for a little kid), and I recently decided to reread it. I enjoyed it as much as—or even more than—I used to. In 1870, on a mountain in Pennsylvania, lives Auntie, Clara, and Maren. Auntie is a healer, who can cure nearly any ailment. Maren is slowly transforming into a mermaid. Clara is just an ordinary girl. Clara and Maren’s favorite tale is the story of three orphans: Clara, delivered to Auntie by a stork. Maren, who appeared in a seashell. And their best friend O’Neill, who was found under an apple tree.
Clara is worried for her sister. Maren’s transformation is getting quicker, and if she doesn’t make it to the ocean in time, she’ll die. So Clara and O’Neill take Maren, and begin the journey. But trouble is lurking, and certain people will do anything to get their hands on a real life mermaid.