As Prince William and Kate prepare to welcome their second child, it's not always smooth sailing for royal heirs and spares

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For royal parents Prince William and Kate, their second child will be a much-loved addition to their family and a sibling for little Prince George.

A life of privilege, comfort and – William and Kate no doubt hope – privacy awaits as the newest addition to the family is raised a prince or princess.

But being the “spare” to the “heir,” as many see it, is not without its pitfalls, if history is any indication. Princess Diana ensured her sons William, 32, and Prince Harry, 30, were brought up as “normally” as possible, and they share a close bond that has arguably not been matched by other heirs and their immediate successors.

That’s not always been the case. “There is definitely a tradition of estrangement between heirs and spares – sublimated envy, perhaps the cause – and the closeness between William and Harry is very unusual,” says respected royals commentator Robert Lacey. The vast fortune guaranteed by the Duchy of Cornwall that goes to the Prince of Wales as heir can be “isolating” from his siblings, Lacey believes.

And, as Lacey points out, it’s not always the first-born who becomes a great, famous monarch. Henry VIII and Elizabeth I and “in our own time, George VI was a younger sibling – to Edward VIII,” Lacey, author of A Brief Life of the Queen, says of the quirks of history. “George VI was written off all his life as an insignificant younger brother, but he saved the monarchy after the abdication, and his fallible qualities were what people responded to during the second World War.”

Brotherly Bond

Apart from sharing a close-knit circle of friends, William and Harry have presented a unified front in their public lives. The two co-founded and lent their names to a foundation, along with Kate, 32, and are the only senior royals to share a patronage of the Henry van Straubenzee Memorial Fund.

As with any parent, William and Kate certainly hope that the future will be smooth sailing for their two children – a challenge that another pair of royal siblings knew all too well.

Queen Elizabeth, now 88, and her late sister, Princess Margaret, had a four-year age gap. But despite being “totally different characters, as different as chalk and cheese, they were very, very close,” says Margaret’s biographer, Christopher Warwick. “There was always a loving bond between the two of them. She and Margaret spoke pretty much every day on the telephone.”

But there were differences in how they were treated, and it raised eyebrows.

“When it was clear that the Queen was heiress presumptive, she had lessons in constitutional history, and I can remember conversations with Princess Margaret when I said, ‘Did you find it surprising you weren’t given the same lessons?’ And she said, ‘I know, it was always a bone of contention,’ ” recalls Warwick, author of Princess Margaret, A Life of Contrasts.

“I said, ‘I bet it was. It was short-sighted because your father was second son and you were one heartbeat behind your sister and weren’t given the further education that was deemed totally relevant to your sister,’ ” Warwick added. “She totally agreed.”

A Growing Family

In the case of Prince George, however, sibling dynamics may very well change should William and Kate decide to have a third child.

Royal observers, for their part, believe that may still be in the cards for the couple.

“For all her skills, one has not seen in Catherine” – herself one of three children – “a burning vocation apart from her wish to support William as a creative consort. So having more children is altogether part of that,” Lacey suggests.

“My sense is that middle and upper class parents in Britain today have more children than less,” he adds. “I would have thought there are more to come.”

For more on the royals, including Princess Kate and her family, pick up the latest issue of PEOPLE, on newsstands now.