Diana Wynne Jones's Magic and Myths Collection (The Game, the Power of Three, Eight Days of Luke, Dogsbody)

Hayley’s parents disappeared when she was a baby. Since then, she has been raised and homeschooled by her grandparents. Grandad is overworked and travels a lot; Grandma is much too strict and never lets her meet any children her own age. When Hayley does something wrong—she is not quite sure what—they pack her off to her aunts in Ireland. To Hayley’s shock, her family is much bigger than she thought; to her delight, the children all play what they call “the game,” where they visit a place called “the mythosphere.” And while she plays the game, Hayley learns more about her own place in the world than she had ever expected.