On Monday, it was confirmed that King Charles had been diagnosed with cancer. The monarch will have plenty of support from his family, including his wife, Queen Camilla, his sons, Prince William and Prince Harry, and his siblings, Princess Anne, Prince Andrew and Prince Edward.
The King is known to have an incredibly close bond with his sister, the Princess Royal, with the monarch even entrusting her to be his Gold-Stick-In-Waiting, a role that is associated with the King's safety during the coronation process.
The monarch's relationship with his brother, Andrew, is increasingly fraught following the Duke of York's alleged behaviour while his relationship with Edward has certainly had its fair shares of ups and downs.
But what do you really know about the monarch's three siblings, well read on below to find out all you need to know…
Princess Anne
Born on 15 August 1950, Anne has become a leading force within the royal family with the Princess Royal regularly named as the 'hardest-working' royal every year. The 73-year-old is currently 17th in line to the throne, although this is mainly down to male-orientated primogeniture rules that meant when Prince Andrew and Prince Edward were born, Anne moved behind them and their children in the order of succession.
Anne has had an incredibly interesting life, with the royal previously dating Andrew Parker-Bowles, the ex-husband of Queen Camilla, was nearly kidnapped back in 1974 and has had two marriages and two children.
In 1968, Anne met Mark Phillips, a lieutenant in the 1st Queen's Dragoon Guards, and the pair would go on to get engaged in 1973. Together, they would welcome two children, Peter and Zara, however, things would not last for the couple with the pair separating in 1989 before finally divorcing in 1992.
Months after their divorce, Anne walked down the aisle again with Sir Timothy Laurence, who was a commander in the Royal Navy at the time. Anne became the first British royal to remarry since Princess Victoria Melita of Saxe-Coburg and Gotha, a granddaughter of Queen Victoria, in 1905.
Alongside her two children, Anne is also a doting grandmother. Her son Peter has welcomed daughters Savannah, 13, and Isla, 11. Meanwhile, her daughter Zara welcomed Mia, ten, Lena, five, and Lucas, two.
Anne lives with Sir Timothy in Gatcombe Park, a country house in Gloucestershire, with Zara also owning a cottage on the property where she lives with husband Mike and their three children. The home has been the base of operations for Anne since 1976.
Anne and Charles have a famously strong relationship, with the Princess Royal essentially being one of the King's right-hand women. Anne has described herself as the "eyes and ears" of the monarchy during an outing and she took on a special honour at the King's coronation, being in charge of his security as his Gold-Stick-in-Waiting.
The royal brother and sister have been seen on many occasions giggling together, whether they be at events hosted at Buckingham Palace, their traditional outing to the Braemar Games or even during the late Queen's Platinum Jubilee celebrations.
Prince Andrew
Andrew is 12 years younger than his older brother, and is currently eighth in line to the throne, but the Duke of York has been in a storm of sexual allegations over the past few years due to his friendship with the late Jeffrey Epstein, who was convicted of sexual trafficking.
The allegations have seen Andrew retreat out of public life, with his late mother stripping him of some of his royal titles, while charities that used to be linked to the Duke have removed their support from him. The scandal also appears to impacted Andrew's relationship with his brother, with the King having reportedly been keen to keep his younger brother out of the public sphere.
However, things have appeared to mellow on that front, with Andrew previously seen with the Prince and Princess of Wales in a car ride to church, and he joined his family at the annual Christmas Day outing to Sandringham.
In 1986, Andrew walked down the aisle with Sarah Ferguson, with the pair going on to welcome two daughters, Princess Beatrice and Princess Eugenie. However, Andrew and Sarah's relationship broke down, with the pair divorcing in 1996. However, Andrew has kept a close bond with his ex-wife with the pair currently living together in the Royal Lodge, where they look after the late Queen's corgis.
Andrew and Sarah are also grandparents with Beatrice being a mum to daughter Sienna, two, while Eugenie has welcomed sons August, two, and Ernest, who is eight months old.
Prince Edward
Prince Edward is the King's youngest brother and was born on 10 March 1964. Edward has enjoyed a relatively low profile, however, the royal still takes on many roles within the royal family.
Edward, who is 14th in line to the throne, found love with Sophie, Duchess of Edinburgh, with the pair walking down the aisle on 19 June 1999. The lovebirds have welcomed two children, Lady Louise Windsor and James, Earl of Wessex, and the pair have managed to keep their teen children mostly out of the spotlight.
Edward and the monarch have had a fraught relationship over the years. The pair were close in their childhood, with sweet photos seeing the then-teenaged Prince playing around with his younger brother, but there have also been times of hardship. In 2001, Edward's former production company allegedly invaded the privacy of Prince William while he was studying, a move which angered Charles. However, the brothers have put their feud behind them, with Edward supporting his older brother during his coronation, with a sweet unseen moment being aired in a BBC documentary.