Almost seven years have passed between the death of Princess Diana in a Paris car crash and the opening of a fountain in her memory in Hyde Park.
The Queen is expected to praise Diana's contribution to the nation
Since she died in 1997 two families - the Windsors and the Spencers - have been separated by painful divisions.
When the Royal Family join Diana's brother Earl Spencer for the opening, it will be their first public appearance together since her funeral.
A speech by the Queen praising Diana's contribution may help heal the rift.
She is expected to recognise the huge loss felt by the nation when the princess died with Dodi Fayed in the crash in the Pont d'Alma tunnel.
BBC royal correspondent Nicholas Witchell said the hostility between the Spencers and the Windsors ignited when Earl Spencer made a speech attacking the Royal Family at Diana's funeral - and has simmered ever since.
At Westminster Abbey in 1997, Earl Spencer paid tribute to his sister and made a promise to her to care for Princes William and Harry.
He said: "We, your blood family, will do all we can to continue the imaginative and loving way in which you were steering these two exceptional young men, so that their souls are not simply immersed by duty and tradition but can sing openly, as you planned."
The memorial has been designed to reflect Diana's joy and grief
The reference to the Spencers as Diana's "blood family" was deeply wounding to the Windsors, our correspondent said.
And in the years since her death the differences between the two families have grown "ever deeper".
The Spencers' decision to open an exhibition at Althorp, the family home and Diana's burial place, was regarded by the Windsors as a tasteless exploitation of her memory, our correspondent said.
Earl Spencer responded that the museum in Northamptonshire was "beyond reproach".
"I am a brother who has lost a sister and brothers who lose sisters don't profit from their death," he said when it opened.
"I've always tried to balance my own ideas against what I think Diana would have wanted."
The furore surrounding the trial of Paul Burrell, the princess's former butler who was accused of stealing her possessions, added to the tensions.
A speech by Earl Spencer at Diana's funeral ignited a family row
The Spencers had pushed for his prosecution, with Diana's sister Lady Sarah McCorquodale and late mother Frances Shand Kydd appearing as witnesses against him.
But the Windsors were reluctant to agree to the trial - and evidence passed on by the Queen eventually led to Mr Burrell's acquittal in November 2002.
The former butler is not expected to be among the guests at the fountain's opening ceremony.
The strains between the two families were apparent again after the death of Mrs Shand Kydd in June this year.
Although Princes William and Harry did attend their grandmother's funeral, her son-in-law Prince Charles was absent.
But the unveiling of the memorial will bring together the Queen, the Duke of Edinburgh, Prince Charles, Diana's sons and the Spencers - and may point to reconciliation in the future.
Our correspondent said: "More than anything it is for William and Harry, princes who are part Windsor, part Spencer, that the two families have finally resolved to draw a line under the past."