Threshold by Angela J. Reynolds
"Angela J. Reynolds' debut novel, for middle-grade readers, takes us to a coastline in Cape Breton where there are crises on dry land and the seals in the water are much more than they seem." -- from the publisher's catalogue
Twelve-year-old Terra is spending the summer in Cape Breton with her aunt Kendall while her mom takes care of her younger brother, Dylan, who has fallen very ill. Terra spends time by the ocean and befriends a mermaid called Minne, who takes her under the sea and introduces her to the Murgelt people. Terra finds an artifact that might belong to them and, despite interference from Alise, the daughter of Kendall's new girlfriend, Terra unravels a family mystery for both the Murgelt and for Alise.
This book is for readers who love magical environments, character-driven stories, fantasy, and a hint of mystery.