The siblings were just toddlers when their mother, the Queen, ascended the throne and they relied on one another for company when their parents were away on royal tours.
WATCH: Queen's children mount vigil around their mother's coffin in tribute
The pair have also been present for key moments in each other's lives, such as Charles' investiture as the Prince of Wales in 1969, Anne's participation in the 1976 Olympics in Montreal, and the recent passing of their mother.
HELLO! takes a look at some of their best moments together…
Princess Anne and King Charles as children
Charles and Anne were pictured playing together outside a miniature house in the grounds of Windsor Castle in 1954.
Princess Anne and King Charles in Malta
Charles lent a helping hand to his little sister as they were reunited with their parents in Malta in 1954 after the Queen and the Duke of Edinburgh's long tour of the Commonwealth.
King Charles and Princess Anne as teenagers
The teenage royal siblings were seen enjoying a stroll together around Windsor Great Park in April 1968.
Charles and Anne began carrying out official duties after finishing their education, and accompanied their parents on royal tours, including Australia and New Zealand.
The King and the Princess Royal are often listed as the two most hard-working royals within the family, carrying out hundreds of engagements each year.
Princess Anne and King Charles horse riding
The pair, pictured in Windsor in 1969, also share a love of horse riding, with Anne being an accomplished equestrian and Charles playing polo competitively until the age of 57.
King Charles and Princess Anne at the Prince of Wales' investiture
Charles arrived at his investiture in 1969 with his sister Anne by his side. The Princess was too young to attend her mother's Coronation in 1953.
Prince Charles and Princess Anne at Royal Ascot
Charles and Anne always look like they have such fun together, particularly at sporting events. The pair looked like they had a good catchup at Royal Ascot in 2012.
King Charles and Princess Anne in Scotland
We love this snap of the pair giggling together at the 2010 Braemar Highland Games. Before her passing, the Queen used to attend the event with her family during the royals' summer break at Balmoral.
King Charles and Princess Anne at the Braemar Highland Games
We couldn't not include this hilarious image. We'd love to know what made the pair laugh so much at the 2017 Braemar Highland Games.
Charles and Anne are both known for their sense of humour, and a clip at a Buckingham Palace event in 2018 showed the King sneaking up on his younger sister to give her a small fright!
King Charles and Princess Anne at St James's Palace
On occasion, the Queen's eldest children carry out joint engagements together, and always look to be having a good time in each other's company.
Here they are attending a reception after presenting the Queen's Anniversary Prizes for higher and further education in February 2022.
King Charles and Princess Anne at the Platinum Jubilee
We'd love to know what had the King in stitches and what Anne said to Charles as the siblings attended the Platinum Pageant over the Queen's Platinum Jubilee weekend in June 2022.
King Charles and Princess Anne at the Queen's funeral
On 19 September 2022, the royal family gathered to bid a final farewell to the late Queen. Princess Anne was a true pillar of support as she walked by Charles’ side during the moving coffin procession towards Windsor Castle.
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