They were the closest of sisters and the best of friends. But when, in a quixotic twist of fate, their uncle Edward VIII decided to abdicate the throne, the dynamic between Elizabeth and Margaret was dramatically altered. Forever more, Margaret would have to curtsey to the sister she called 'Lillibet'.

And bow to her wishes. Elizabeth would always look upon her younger sister's antics with a kind of stoical amusement but Margaret's struggle to find a place and position inside the royal system - and her fraught relationship with its expectations - was often a source of tension. Famously, the Queen had to inform Margaret that the Church and government would not countenance her marrying a divorcee, Group Captain Peter Townsend, forcing Margaret to choose between keeping her title and royal allowances or her divorcee lover.

From the idyll of their cloistered early life, through their hidden wartime lives, into the divergent paths they took following their father's death and Elizabeth's ascension to the throne, this book explores their relationship over the years. Andrew Morton, renowned bestselling author of Diana: Her True Story, offers unique insight into these two drastically different sisters - one resigned to duty and responsibility, the other resistant to it - and the lasting impact they have had on the Crown, the royal family and the way it has adapted to the changing mores of the twentieth century.