Prince Harry and Prince William cannot "do proper honor to their mother's memory" without a reconciliation at a Princess Diana statue unveiling, historian Robert Lacey told Newsweek.
The royal brothers' relationship has been torn apart by bullying allegations and claims of insensitivity towards Meghan Markle, the British Monarchy's first royal of color in living memory.
However, on Thursday they will together unveil a statue in their mother's memory at The Sunken Garden of Kensington Palace, a place Princess Diana went for moments of quiet contemplation.
Historian Robert Lacey chronicles the royal rift in his bestselling biography Battle of Brothers and says this week must mark a turning point for the siblings if the commemoration of Diana is to have any meaning.
He told Newsweek: "Maybe there'll be some clicking of the fingers and waving of the wand on Thursday.
"I don't see how they can do proper honor to their mother's memory without, not a total reconciliation, but a meaningful gesture of homage to her and to each other.
"Otherwise what's the world going to say? If they come out and make two wooden speeches and then go their separate ways what meaning will that ceremony have and what conclusion will the world draw?"
Harry and William will gather in the same place where four years ago The Sunken Garden was redesigned as a white garden to mark the 20th anniversary of Diana's death.
Then the bullying allegations, the royal exit, the Oprah Winfrey interview and so much of their rivalry was yet to happen.
This time their wounds have been laid bare in Lacey's book, among other places, but they will be surrounded by the people who helped offer emotional support to their mother.
Among them is Charles Spencer, the princess' brother who gave a eulogy at her 1997 funeral, famously vowing that her "blood family" would steer Harry and William "so that their souls are not simply immersed by duty and tradition but can sing openly as you planned."
Lacey has described how Prince William asked Earl Spencer to speak to Harry early on in his relationship with Meghan in the hope of patching up disagreements.
He told Newsweek: "Charles Spencer was called in earlier, right at the beginning of the courtship [between Harry and Meghan] and it didn't work.
"Harry rebuffed him. This [statue unveiling] will be an essentially Spencer occasion with the addition of Julia Samuel who acted as a counselor to Diana and has been acknowledged by Harry and Meghan in the Oprah interview as being a source of comfort to them.
"There's nothing really more the palace or the other friends can do."
Charles Spencer with William and Harry
In a pre-recorded speech for charity The Diana Award, Prince Harry said yesterday: "Later this week, my brother and I are recognizing what would have been our mom's 60th birthday, and she would be so proud of you all for living an authentic life with purpose and with compassion for others.
"Our mom believed that young people have the power to change the world. She believed in your strength because she saw it day in and day out, and in the faces of young people exactly like you, she witnessed a boundless enthusiasm and passion."
He closed the speech by saying: "Stand up for what you believe in and trust that when you live by truth and in service to others, people will see that, just as they did with my mom."
Those final remarks appear to echo a justification he has offered for giving the March 7 tell-all Oprah Winfrey interview.
He told his Apple TV docuseries The Me You Can't See: "I like to think that we were able to speak truth in the most compassionate way possible, thereby leaving an opening for reconciliation and healing.
"The interview was about being real, being authentic and hopefully sharing an experience that we know is incredibly relatable to a lot of people around the world despite our unique privileged position."
The words in The Diana Awards speech could be interpreted as a renewal of this defense of his and Meghan's decision to live by truth in their bombshell disclosures about royal life.
However, there is one subject on which Lacey says the brothers are united in wishing to preserve their mother's memory.
He said Prince Charles has been lobbying behind the scenes for support in changing official royal policy so that when he becomes king his second wife, Camilla, Duchess of Cornwall, takes the title queen, originally destined for Diana.
Camilla has not been named Princess of Wales out of respect for William and Harry's mother who held the title.
And the royal family's official position is that she will be named Princess Consort rather than Queen Consort.
However, the Prince of Wales has other ideas and has met opposition to his campaign from Harry and William, according to Lacey.
He told Newsweek: "If Uncle Charles Spencer can't handle reconciliation, who can? Father Charles clearly can't. That's been obvious publicly. Privately I discovered both brothers are absolutely sick to the back teeth of Charles trying to negotiate full Queen status for Camilla."
A Kensington Palace spokesperson last week said: "Prince William and Prince Harry will attend a small event to mark the unveiling of a statue they commissioned of their mother, Diana, Princess of Wales, in the Sunken Garden at Kensington Palace on Thursday 1st July.
"In addition to close family of Diana, Princess of Wales, members of the statue committee, the sculptor Ian Rank-Broadley, and garden designer, Pip Morrison, will also be present."
