In “Royal Sisters”, Anne Edwards, author of the bestselling Vivien Leigh: A Biography and Matriarch: Queen Mary and the House of Windsor, has written the first dual biography of Elizabeth, the princess who was to become Queen, and her younger sister, Margaret, who was to be her subject. From birth to maturity, they were the stuff of which dreams are made. "I'm three and you're four," the future Queen, then a child, imperiously informed her sister....