Life is going along fine for twins Gus (a girl) and Leo (a boy) in their tiny coastal Maine town until their mother becomes ill, falling into a sudden coma that perplexes the doctors. Comments by their grieving father lead the twins to believe that Mom’s condition isn’t the result of natural causes, and when a strange man with the ability to shapeshift into a sea mink turns up at their house, the extent of their mother’s secrets becomes clear. The man/mink informs them that their mother is actually one of the Folk—specifically, a selkie—and that as her kids, Gus, Leo, and their little sister Ila are also magical creatures with the ability to transform into seals (and in Ila’s case, also a fox). An age-old evil now threatens the Folk and the three siblings must help their previously unknown grandmother defeat the threat if they are to save their mother. There are quiet echoes of Susan Cooper and C. S. Lewis here in both mythology and structure, and Raabe manages the difficult feat of balancing the family drama and the epic fantasy with surprising ease. The affection among the siblings is particularly well drawn; the third-person narration focalizes through Gus, who feels responsible for her more passive brother, and through little Ila, who struggles with feeling left out of the twins’ inherent bond. Vivid imagery makes the underwater scenes utterly captivating, as the siblings in seal form frolic with dolphins, battle off great white sharks, and, of course, eventually beat the bad guy. A cozy fantasy with a few deep-sea thrills, this would make a fine family or classroom readaloud as well.