After what had been one of the most tortuous weeks in modern royal history, they sounded a welcome note of family harmony.

Daubed in bright colours and expressing love and affection for the grandmother they never knew, the Mother’s’ Day cards made for “Granny Diana” by Prince George, Princess Charlotte and Prince Louis served as a searing reminder of all their father has lost.

Posted on the Kensington Palace official Instagram account on Mothering Sunday – a week on from the Sussexes’ fateful interview with Oprah Winfrey – the hand-written missives, adorned with kisses, were as poignant as the revelation that Prince William’s children write loving messages to the woman the world knew as the “Queen of Hearts” every year. Charlotte’s card read: “Papa is missing you.”

Combined with the Duchess of Cambridge’s unexpected attendance at Saturday’s vigil for Sarah Everard, the 33-year-old marketing manager kidnapped and killed on her way home in London, the messages added to the impression that the Cambridges are taking back control of their own narrative in the face of the Sussexes’ “truth”.

Against the backdrop of their acrimonious clash with the Royal family – with Meghan having accused Kate of making her cry, amid allegations of racism in the Royal family and Harry’s description of his father and brother as “trapped” – was this the Cambridges’ subtle way of tipping the balance?

As respected royal biographer Penny Junor puts it: “I think it was very much William trying to say: ‘I am Diana’s son, too.’”

During the two-hour tell-all with Oprah, Harry repeatedly invoked their late mother, suggesting that she would have been angry at the way he and Meghan had been treated and even going so far as to state of their departure: “I think she saw it coming.”

Yet that recollection of events is unlikely to be shared by William, 38, who has always taken issue with his mother’s reputation as a “royal rebel” when she was the epitome of public service, spending 16 years stalwartly representing Queen and country despite her inner turmoil.

“At the moment Harry seems to have taken ownership of the whole story,” adds Junor. “He is calling the shots on what this family is all about and invoking Diana. But what he’s actually done is effectively put a bomb under William’s future. He’s done such damage to his family.”

Shortly after the Cambridge children’s touching cards were posted online, it emerged that Prince Harry had arranged for flowers to be placed on his mother’s grave in the grounds of Althorp Park – a reminder of the different ways in which the brothers are both still wrestling with Diana’s legacy.

Intriguingly, though, the Cambridges’ recent candour when it comes to publishing images of their children – along with their heartrending artwork – does appear to borrow from the Sussex playbook of drawing back the curtain on the window to their souls.

The previously intensely private couple have been far more open since Harry, 36, and Meghan, 39, became a couple, regularly sharing snaps from the family photo album while laying bare their own personal feelings about issues such as parenting and mental health.

Since having her three children, the 39-year-old Duchess of Cambridge has appeared to spread her wings, spearheading a landmark study on early years learning while carrying out other solo projects such as designing a garden at the RHS Chelsea Flower show in 2019.

Her unexpected presence at Saturday’s “banned” vigil on Clapham Common, when she let it be known that she she “remembers what it was like to walk around London at night before she was married”, marked a real watershed for the once-passive Catherine, asserting her credentials as one of the monarchy’s most influential members.

Junor believes that the couple “are acutely aware of the following the Sussexes have got” and are “rightly trying to emulate it”.

The author, 71, explains: “People my age are bemused and appalled by what Harry and Meghan said on Oprah but younger people, who don’t give two hoots about monarchy, sided with the Sussexes and they are the future.

“Maybe the Cambridges are learning, just as the Royal family did when Diana came into the family and started doing things in a different way, that there is traction to be had in being a little more open with the public.”

Both brothers must have in mind the public scrutiny that will come with their attendance at the unveiling of the statue of their mother on what would have been her 60th birthday on July 1.

All eyes will be on the estranged siblings as they are reunited, possibly for the first time in 15 months, in Kensington Palace Gardens.

Despite all that has been said and done, and with William admitting he had not even spoken to his brother on Thursday, three days after the interview aired in the UK, royal aides insist it would be “unthinkable” for either to pull out.

Royal watchers remain perplexed as to how the Palace are going to orchestrate what could prove to be a very awkward engagement with the world’s press looking on.

“Part of me thinks that the relationship is possibly irreparable,” Junot says. “We know how determined William can be and Harry and Meghan have not only trashed his wife but accused the Royal family of racism. It’s the most damaging thing that anyone could say [about] anyone else. It is hard to see how they come back from this.”

Behind palace gates, however, insiders insist the Cambridges remain “hopeful of a reconciliation”, saying: “What they really want is peace.” Evoking the spirit of the Queen’s statement, in which she said the “serious” and “concerning” issues raised by Harry and Meghan would be “addressed privately”, there appears little desire for a protracted transatlantic slanging match, although as one source put it: “The trust has been lost.”

It is perhaps indicative of the depth of the tensions between the two brothers that one of the reasons the statue project stalled, after first being announced in January 2017, was because they disagreed about the design.

The two princes – who were just 15 and 13 when Diana was killed in a Paris car crash on August 31, 1997 – commissioned the statue of the woman who they said “touched so many lives” after the £3.6 million Diana Memorial Fountain in Hyde Park was beset with problems. The Princess’s grave on an island at Althorp cannot be accessed by the public, and the royal brothers had long felt there was no “fitting or lasting tribute” to her life. But after commissioning sculptor Ian Rank-Broadley, whose likeness of the Queen has appeared on Commonwealth coins since 1998, it is thought Diana’s sons struggled to agree on what the statue should look like.

Eighteen months passed, and in July 2019, amid reports he was “barely on speaking terms” with Harry, William reportedly told fans gathered outside Kensington Palace (on what would have been his mother’s 58th birthday) that the statue would materialise “soon, very soon”.

It was only in August last year, five months after the Sussexes had moved to Los Angeles, that Kensington Palace announced the big reveal would finally be taking place in the Sunken Garden this summer.

A rare joint statement on behalf of both princes announced: “The statue was commissioned to mark the 20th anniversary of her death and recognise her positive impact in the UK and around the world.

“The Princes hope that the statue will help all those who visit Kensington Palace to reflect on their mother’s life and her legacy.”

It also now gives them a deadline to ensure that the unveiling is not overshadowed by sibling rivalry, no matter how deep-seated or difficult to resolve it may seem to have become.

For, regardless of what Diana might have made of “Megxit”, the woman who William describes as having “smothered me and Harry in love” would surely want her sons to rediscover the special bond she fostered between them, as brothers in arms.