She was the star of the King’s Boxing Day documentary with her candid commentary, shrewd insight and propensity to make her brother – and no doubt the viewers - laugh.

Who else would greet the newly crowned Charles with ‘Hello, Old Bean’?

It was Princess Anne who explained to BBC viewers just how hard it had been for her brother to prepare himself for the Coronation, despite a lifetime of waiting in the wings.

And it was Anne who summed up Charles's feelings after the ceremony as like those of ‘an actor who comes off stage having done a performance that they really put a lot into. It’s that kind of relief’.

Increasingly described as the King’s ‘right-hand woman’, the Princess Royal is emerging as a figure of shared understanding and trust.

'Hello old bean'  - the Princess Royal greets her newly crowned big brother, King Charles III

Anne, Princess Royal pictured in a bicorn hat with towering feather during the Coronation of King Charles III

Princess Anne tells officers of the Household Division that the monarchy is safe in Charle's hands

Next week she jets to Sri Lanka for the first Royal tour of 2024. The three-day trip, made at the request of the Foreign Office, marks 75 years of diplomatic relations with the United Kingdom.

Such close working has not always been so evident, however. 

Through much of their adult lives, Charles, now 75, and little sister Anne, 73, led rather separate lives and there was a time when relations were thought to be decidedly cool.

Their respective country homes, Highgrove House and Gatcombe Park, might stand just six miles apart in the Cotswolds, but there was  little evidence of family visits.

They have different interests and approaches, of course, as their very different houses might suggest.

Manicured Highgrove with its lovingly tended gardens is a living work of art.

Princess Anne’s Gatcombe is a rambling shrine to all things equine – part stables,  part family compound, with children, grandchildren and even exes living on the grounds.

Born less than two years apart, Anne was more always extroverted than Charles.

Thought of as close to Prince Philip, and famous for her brusque, non-nonsense delivery, she was once described as being ‘very much her father’s daughter - in a way that Charles could never be his father’s son.’

They have some things in common, too, not least a sense of humour - Charles is surprisingly funny - and devotion to sheer hard work.

Analyses of last year’s royal engagements disagree as to which was the hardest-working HRH, but on this they concur: it was either the King or it was the Princess Royal.

Anne carried out 457 engagements while Charles did 425 according to The Daily Telegraph.

The Times says that Charles conducted 516 engagements at home and abroad with the Princess Royal completing 410.

There’s been little time for house calls.

For all their differences, Charles and Anne are yoked together by circumstance and duty – a bond which is now as clearly on view as it has ever been.

King Charles III and Anne, Princess Royal attend the Committal Service for Queen Elizabeth II at St George's Chapel, windsor

Anne, Princess Royal, publisher Sir Nicholas Coleridge (c) and King Charles III at last year's  Braemar games 

Prince Charles, Prince of Wales, Queen Elizabeth II and Princess Anne, Princes Royal attend the 2018 Braemar Highland Gathering

The Princess Royal and Prince Charles, Prince of Wales attend the funeral of Patricia Knatchbull, daughter of Louis Mountbatten at St Paul's Knightsbridge  in 2017

Prince Charles, Prince of Wales and Princess Anne, Princess Royal athe Platinum Pageant on The Mall in 2022

Four generations of the British Monarchy pictured together in Buckingham Palace after Princess Anne's christening in 1950. From the left, Queen Mary, King George VI, Queen Elizabeth II, Prince Philip, Prince Charles and Queen Elizabeth, the Queen Mother

Princess Anne was known as a 'daddy's girl'  seen here with her hand on Prince Philip's mouth

Princess Anne did not attend the Coronation, although Prince Charles was taken part way through. But the two-year-old came out on to the balcony afterwards and stood next to her big brother

Prince Charles aged 6 and Princess Anne, 4, pose with their mother in 1954

Duke of Edinburgh, Princess Anne, Prince Charles and Queen Elizabeth II, pictured looking down from the East Terrace Wall of Windsor Castle in 1959

Princess Anne, Prince Charles, Tony Snowdon and his new sister-in-law Queen Elizabeth II watch the Badminton Horse Trials in 1960

Princess Anne, Prince Charles, Prince Philip, Duke of Edinburgh and Queen Elizabeth II meet Richard Nixon, President of the United States in February 1969

Prince Charles side by side with sister Princess Anne in February 1970

Of the late Queen’s four children, only two knew their mother before life was transformed by her accession to the throne.

Accident or otherwise, it was Charles and Anne who were by Elizabeth's side in her final moments at Balmoral, not the younger ones.

A lifetime earlier when she was a blonde-haired two-year-old, Anne had been left  back at the Palace when Charles, the future King, was taken to see the Coronation of their mother.

But she will know everything there is to know about what her big brother went through in that extraordinary 70-year wait to play his part – and what he now faces in the time remaining.