Nimmo (""The Snow Spider"" trilogy) turns from Welsh legend to kelpie lore, weaving a delicate strand of fantasy into a story about siblings who discover their true identities while following an inborn call to save sea birds imperiled by environmental disasters. Left with a warmhearted aunt and a bitterly antagonistic grandmother--neither of whom they remember meeting before--while their mother (Leah) honeymoons with a new husband, Ned (11) and Nell (8) make friends with charismatic Arion, who seems to rise from the sea and with whom they are mysteriously at ease; he takes them to rescue sea birds after an oil spill, leading to a tragic confrontation between Nell and her witchlike grandmother. Finally, it turns out that Arion, who believes himself to be half-kelpie, is actually their father, and their mother was not Leah but ""Ultramarine,"" who died on a mission to save sea birds, and who was sister to Leah's first husband. As usual, the realistic part of Nimmo's story is the most interesting: Ned's protective role toward his withdrawn sister, their anguish on learning that Leah is not their birth mother, the bereaved grandmother's dementia. But the environmental plea is not as well integrated as in Ruth Park's My Sister Sif (1991), which dealt more skillfully and imaginatively with similar themes; still, this also holds attention with its aura of magic and mystery, appealing characters, and intricate, unusual plotting.