Prince William and Prince Harry have a "heavy burden of expectation and responsibility", Diana's former private secretary has said.
Patrick Jephson said the two brothers are shaping their own lives and following different interpretations of royal duty.
But if on one side this means they have many responsibilities for the future, it also represents an opportunity for the Duke of Cambridge and the Duke of Sussex.
The two brothers came together on July 1 to unveil a statue for their late mum Diana at Kensington Palace's Sunken Garden.
They were seen smiling and chatting despite their ongoing rift during an intimate family ceremony.
Writing in the Daily Mail after the event, Mr Jephson said that both Harry and William are now older than their mum was when she passed away - and this means they can have opportunities she did not have.
He wrote: "So much for the past, what of the future? As the Queen said in 2004 at the opening of the Diana Memorial Fountain: ‘The departed live on in those they have helped to shape in life.’
"Today, Diana’s sons are shaping their own lives and following their own interpretations of Royal duty. Time flies: they are both older than she was when she died. For them, every new morning is one more than she was allowed.
"That’s a heavy burden of expectation and responsibility. But it’s an opportunity, too… as they surely don’t need reminding."
Mr Jephson also commented on the statue made to honour Diana, saying sculptor Ian Rank-Broadley had an impossible task.
He said: "Diana was not an easy person to define and nor is her legacy.
"If our memories and appreciation of Diana begin and end with her love of children – as the committee appears to be encouraging us to believe – then we do both her and ourselves a disservice."
Patrick Jephson was Equerry and Private Secretary to the Princess of Wales between 1988 to 1996.
He witnessed the breakdown of Diana's marriage to Prince Charles and the couple’s divorce.
Earlier this week, royal author Robert Lacey said that during the years after their mum's death, Harry and William have always found ways to keep Diana's legacy alive.
He told People : "In the past quarter of a century, they have placed their mother right at the heart of the royal family."
Mr Lacey added this is "something the Queen and Prince Charles certainly wouldn't have done", but the two brothers "have done it together".
The royal author and historian said Harry and William have admitted there are differences between them, but found a "constructive way of dealing" with them.
