A new biography has explored the complex relationship that Queen Elizabeth II shared with her late sister, Princess Margaret.

Royal biographer Andrew Morton, the author of the 1992 bestseller "Diana: Her True Story," writes about the royal siblings in his new book "Elizabeth & Margaret: the Intimate World of the Windsor Sisters." In a conversation with People magazine about the biography, Morton said that Princess Margaret, who passed away in 2002 at the age of 71, "was someone who understood the Queen in a way no one else could."

The Queen has often been hailed for being dutiful, while her only sibling was known to be free-spirited. Their contrasting personalities can be best understood from a famous statement by their father, King George VI, who said: "Lilibet is my pride; Margaret is my joy."

Despite the vast difference between their temperaments, and the rifts they had over the years, the sisters shared a special bond and were fiercely loyal to each other. Morton says about the royals: "They knew each other intimately from the day they were born. There is a unique intimacy between two siblings brought up together, brought up royal together, that is absolutely fascinating."

The author also noted that there is a unique similarity between the differences the Queen had with her sister and the differences between the monarch's grandsons, Prince William and Prince Harry. Although the two pairs of siblings are separated by two generations, they have the same position in the British monarchy- "the heir and the spare."

"The parallels between William and Harry and Elizabeth and Margaret are there to be seen. In both cases you have one sibling who pushes boundaries, while the other is more cautious," Morton said. He pointed out that there is one key difference as well, since "Harry is being far more proactive in using his name and popularity to advance causes in a way Margaret never did."

"She much preferred to be carousing until 4 in the morning," the author said.

In his new book, which hit the shelves on March 30, the author has also mentioned some remarks reportedly made by the two sisters about each other. According to the book, a young Elizabeth once told her governess that "Margaret always wants what I've got," while Margaret said in her fifties that she was "still playing second best after all these years, I guess I'll be second best to my grave," reports Vogue.

"Both sisters were universally known and almost constantly surrounded by people, yet in so many ways they remained indecipherable to everyone but each other. It was from this position of magnificent isolation that the sisters formed their inseparable, intuitive bond," Morton writes in the book.

Princess Diana
4 August 1990: The Queen, Princess Diana and Princess Margaret dress in vibrant floral designs for the celebrations of the Queen Mother's 90th birthday Tim Graham/Getty Images