Princess Diana was adamant that her sons, Prince William and Prince Harry, should be there for one another, which makes their estrangement so painful today.

“Diana always used to say she had two boys for a reason — the younger would be there to support the older in the lonely task as future king,” Diana’s biographer Andrew Morton tells PEOPLE in this week’s exclusive cover story.

“There is no doubt Diana would have tried to act as a peacemaker between them,” says Morton, whose latest book, Winston and the Windsors, is out in October. “If she had been around, they would have worked things out in a different way.”

Twenty-eight years after Princess Diana’s death, William and Harry remain devoted to their mother — yet their fractured relationship shows no signs of healing.

The Prince of Wales, 43, and Duke of Sussex, 40, grew up sharing a royal life and experience only they can understand, weathering the scandalous divorce of their parents, Diana and the future King Charles, and enduring the unbearable pain of losing their mother after a car crash in Paris on Aug. 31, 1997, when they were 15 and 12.

But now, they’re worlds apart.

While Harry has spoken openly about his hope for reconciliation with his family, insiders say his calls and messages to William have gone unanswered, in a separation that extends to the next generation.

​​Prince William and Kate Middleton are raising Prince George, 12, Princess Charlotte, 10, and Prince Louis, 7, in Windsor. Across the Atlantic, Prince Harry and Meghan Markle’s children, Prince Archie, 6, and Princess Lilibet, 4, are growing up in Montecito, California. The cousins have no known relationship amid a familial rift, which exploded into public view in 2020 when the Duke and Duchess of Sussex stepped back from royal duties.

“Things were said that sparked the initial rift, and it’s never healed,” says Morton.

Despite the gulf between them, both brothers remain united in one shared vow: to honor their mother’s memory.

William has followed her lead in his work to help the homeless, continuing with a cause Diana introduced her sons to.

For his part, Harry found his calling in supporting young people affected by AIDS in Southern Africa, following in his late mother’s footsteps. It’s a commitment the Duke of Sussex will continue after stepping back from Sentebale following a dispute with the chair of the charity he co-founded in 2006.

At home, both of Princess Diana’s sons have created lives grounded in a shared commitment to providing their children with a genuinely authentic upbringing — “and that is pure Diana,” says historian Amanda Foreman.

Though silence now divides them, her influence remains a strong bond that still shapes their every step.

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“This is the sadness of it — they aren’t supporting each other like they should be,” says a source close to the royal household. “That’s what any mother would want — that they are there for each other.”