Clara and her adopted sister Maren live with their Parental Substitute Auntie Verity and their pet wyvern Osbert in a cottage on Llanfair Mountain, Pennsylvania in the early 1870s. They have always known that Maren comes from the sea, and at the age of sixteen she starts to transform into a mermaid. If she can't reach the ocean, she will sicken and die. But the journey from Llanfair Mountain to the seashore is long and dangerous.
The Mermaid's Sister contains examples of:
- Bathtub Mermaid: When Maren's legs fully turn into a tail, Auntie buys a large metal tub, a luxury in 1871. She lives in that tub for several months.
- Body to Jewel: When Maren cries, her tears turn into pearls.
- Cannot Tell a Lie: Auntie's mother was Faerie, so she's incapable of saying or even thinking a lie.
- Delivery Stork: Auntie tells Clara that she was delivered by a stork just three days after Maren arrived. Clara is afraid she'll turn into a stork. It turns out she was actually found on the steps of an abandoned orphanage by Scarff, whose middle name, Corraghrain, means stork. Auntie thought Clara would be jealous if she wasn't delivered through magic like Maren was.
- Doorstop Baby:
- One October night, Auntie heard a knock at the door. When she opened it, there was nothing there but a giant conch shell on the path. Auntie took it in and set it on the table. When she tipped it, baby Maren fell out.
- The girls' friend O'Neill was found by a priest in a basket under an apple tree in a churchyard. The priest was too old to raise a child and didn't want to send him to the Orphanage of Fear, so he gave him to the traveling merchant Scarff. Scarff took O'Neill to Llanfair Mountain, wanting to give the girls a brother, but O'Neill cried whenever he was out of Scarff's sight, so Scarff took him on his travels and raised him as his son.
- Groin Attack: When Clara is being forced to work in Dr. Phipps' show, Soraya dresses her up as "Princess Hatsumi" and tells her to stand in the Gallery of Wonders pretending not to speak English. When a customer sexually harasses her, she knees him in the groin. Soraya warns her not to assault a paying customer again.
- Healing Shiv: Shortly after Maren, Clara, and O'Neill set off on their journey, Scarff uses Osbert to deliver them a magic dagger which heals wounds and can only be used once. After O'Neill is shot in the chest, he begs Clara to use the dagger on Maren, who is dying from being away from the ocean for too long, but she knows she won't be able to transport Maren without his help, so she traces the wound with the dagger, which heals the damage in seconds.
- Hide Your Otherness: During the first year or so of Maren's transformation, she can still go out in public, wearing gloves to hide her webbed hands while her scales are all under her clothes. After her feet turn into fins, this becomes impossible, but for a few months she can still receive visitors as long as her body is hidden under a blanket.
- Incredible Shrinking Man: Maren becomes smaller and smaller the longer she is away from the ocean. By the end of their journey, she fits in a two-quart jar.
- Masquerade: The protagonists hide Auntie's faerie nature, Maren's transformation, and Osbert's existence from everyone except Scarff and O'Neill for fear of what the townspeople might do to them if they knew about magic.
- Painful Transformation: Maren regularly suffers pain in her legs for months. Once they fully combine, the pain mostly goes away.
- Snake Oil Salesman: While Maren can still walk, she and Clara go to see a traveling show, where a man named Dr. Phipps sells a variety of mysterious concoctions that he claims can cure every known ailment. Maren and Clara have been warned not to buy anything from him, but some of the other customers get suckered. Over the next few days, many people get sick from taking the elixirs. Auntie makes herbal remedies to help them.
- The Speechless: After her physical transformation is complete, Maren loses the ability to speak. She communicates through gestures. She regains her voice once she is immersed in seawater.
- Superhuman Trafficking: Maren, Clara, and O'Neill are captured by Dr. Phipps, his wife Soraya, and their son Jasper, who keep Maren in a jar and put her on display to be gawked at by customers.
