"We are stuck with each other for the rest of our lives." That's how Prince Harry put it in 2018 when asked about his relationship with William.
Since then so much has shifted. They may be stuck with each other - but they are currently heading in very different directions.
The unveiling of the statue comes at a time of family turmoil. William and Harry will put on a united front but the close relationship between them is broken.
Sources say communications have been minimal - with deep upset and mistrust on both sides.
Yes, there was sibling rivalry. Yes, there was teasing. And yes, there have been arguments over the years.
But what strikes you is the obvious connection they had.
There is a shared grief, a shared recovery, and a clear love and affection for one another in everything they said. Yet what seemed an unbreakable bond has shattered.
It is complicated and personal - with much of it played out in public. There have been conflicts over Prince Harry's wife Meghan, staff, public life, duty and future direction.
It has created a deep and bruising divide that is proving hard to fix.
Robert Lacey has examined what has gone wrong in his book Battle of Brothers. "Our British royal family is not supposed to be a perfect family," he said.
"So the fact that the brothers are at odds at the moment isn't necessarily the end of the value or the importance of the Royal Family. If somehow out of this split comes some sort of reconciliation that will presumably hold a lesson for all of us."
But reconciliation is proving hard to find. The wounds are deep. A resolution will take time, I'm told.
There is unity, though, over the statue to their mother and its place in honouring her legacy. William and Harry commissioned it and have driven the project forward.
"I think unveiling the statue will be hugely emotional," said Tessy Ojo, the chief executive of the Diana Award.
"It will capture the thoughts of her sons around her and that will be so meaningful. We will see her through their eyes."
Through his lens, the Sun's royal photographer, Arthur Edwards, has been capturing the Royal Family on camera for more than 45 years.
He has seen it all - the affection for Diana and the love and sympathy for her sons following her death. For him, the statue unveiling brings genuine sadness.
"Let's just put it this way," he told me. "If that doesn't break the ice between them, nothing will.
"This is their mother, the person they love more than anybody else in the world… and if they can't realise what a big moment this is and just shake hands or just say something lovely to each other then what do you do? Just don't hold your breath."
The sight of William and Harry alongside the statue of their mother - within the grounds of their childhood home, Kensington Palace - will be evocative.
The influence of Princess Diana still impacts the modern Royal Family and the statue will be recognition of that - and a new place for royal pilgrimage in London.
Those who know William and Harry hope this moment will offer more - and that, under the gaze of their mother, they will cast off their rancour and frustration, and begin to heal their troubled relationship.