"There is a unique intimacy between two siblings brought up together, brought up royal together, that is fascinating," says author Andrew Morton
princess margaret and queen elizabeth
Princess Margaret and Queen Elizabeth in 1996
| Credit: Richard Gillard/Camera Press/Redux

As children, Queen Elizabeth — then a princess nicknamed "Lilibet" — and her younger sister, Princess Margaret, were famously described by their father King George VI: "Lilibet is my pride; Margaret is my joy."

The contrast between the sisters' temperaments — dutiful Elizabeth and free-spirited Margaret — would only become more pronounced as they grew up, a theme royal biographer Andrew Morton explores in his new book, Elizabeth & Margaret: the Intimate World of the Windsor Sisters.

Yet despite their deep differences, the sisters were fiercely loyal to each other. Margaret — who died in 2002 at age 71 — "was someone who understood the Queen in a way no one else could," Morton tells PEOPLE in this week's issue. "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."

elizabeth and margaret the intimate world of the windsor sisters by andrew morton
Andrew Morton's new book, Elizabeth & Margaret.

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There are also unique tensions, which have played out recently in the rift between Elizabeth's grandsons Prince William, 38, and Prince Harry, 36. Although they are separated by two generations, "the parallels between William and Harry and Elizabeth and Margaret are there to be seen," says Morton, who authored the blockbuster 1992 biography Diana: Her True Story, for which Princess Diana was later revealed to be the primary source.

princess margaret and queen elizabeth in the 1940s
Elizabeth and Margaret circa 1942.
| Credit: Studio Lisa/Getty

For more from royal biographer Andrew Morton, as well as an excerpt from his new book, pick up this week's issue of PEOPLE, on newsstands Friday

"In both cases you have one sibling who pushes boundaries," says Morton, "while the other is more cautious."

And while both sets of siblings were popularly labeled "the heir and the spare" for their diverging roles within the monarchy, Morton points out at least one key difference.

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Queen Elizabeth ll and her sister Princess Margaret attend the Epsom Derby
Queen Elizabeth and Princess Margaret at the Epsom Derby in 1979.
| Credit: Anwar Hussein/Getty Images

"Harry is being far more proactive in using his name and popularity to advance causes in a way Margaret never did," says the author. "She much preferred to be carousing until 4 in the morning."