The Game by A.S. Byatt

The Game is a lush and disturbing novel portraying a sibling rivalry which compels the reader to reconsider the uses and misuses of imagination. when they were little girls, Cassandra and Julia played a game in which they entered an alternate world modeled on the landscapes of Arthurian romance. Now the sisters are grown, and hostile strangers—until a figure from their past, a man they once both loved and suffered over, reenters their lives.