The brothers dedicated a statue to their mother at Kensington Palace on July 1, which would have been her 60th birthday

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Royal watchers had their eyes closely trained on Prince William and Prince Harry last Thursday, seeking signs alternately of tension or tenderness as the brothers unveiled a statue they commissioned four years ago to honor their late mother, Princess Diana.

But for the brothers, the day was all about Diana. Despite disagreements and relocations in recent years, the brothers have always found their way back together in service of keeping their mother's legacy alive in the years since her death in a Paris car crash in 1997.

"In the past quarter of a century, they have placed their mother right at the heart of the royal family," historian Robert Lacey, author of the bestseller Battle of Brothers: William & Harry: The Inside Story of a Family in Tumult, tells PEOPLE in this week's issue.

It's "something the Queen and Prince Charles certainly wouldn't have done," continues Lacey, but William and Harry "have done it together."

The Duke of Cambridge and Duke of Sussex arrive for the unveiling of a statue they commissioned of their mother Diana, Princess of Wales
Prince William and Prince Harry unveil a statue of their mother, Princess Diana, at Kensington Gardens.
| Credit: Alamy Stock Photo
The Duke of Cambridge and Duke of Sussex arrive for the unveiling of a statue they commissioned of their mother Diana, Princess of Wales
Prince William and Prince Harry
| Credit: Alamy Stock Photo

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At Thursday's ceremony, the brothers decided to forego individual speeches and instead issued a joint statement that maintained focus on their mother: "Today, on what would have been our mother's 60th birthday, we remember her love, strength and character, qualities that made her a force for good around the world, changing countless lives for the better," William, 39, and Harry, 36, said.

They added, "Every day we wish she were still with us."

Lacey previously told PEOPLE that the brothers have "demonstrated a constructive way of dealing with differences."

"[They were] not sweeping [their differences] under the carpet," adds Lacey, "but acknowledging more important things and doing that in a way that warmed everyone's heart."

Indeed, close friends and family of Diana gathered at a scaled-back but deeply meaningful ceremony to celebrate her life and work. One close friend of Diana told PEOPLE that William and Harry honored their mother not just with the statue but also with the spirit of the day itself, saying, "It was their memento, and they were very dignified about it."

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For his part, sculptor Ian Rank-Broadley said at Thursday's ceremony that he hopes the tribute gives the princes "solace" when they visit: "Perhaps in the evening when the grounds are shut, they could come here for a moment of quiet reflection. I hope that will give them some sort of comfort."