Taking to Twitter on Thursday, the 57-year-old shared a beautiful throwback photo from their childhood which showed the siblings sat outside together during a warm day.
WATCH: Charles Spencer shares stunning addition to family home Althorp House
"Some bonds go back a very long way," he simply wrote. The heartwarming picture was inundated with messages from royal fans.
"That’s a lovely photo," remarked one, while another said: "How thoughtful she looks even at this age."
Charles grew up with his three older sisters: Diana, Sarah, and Jane, and is now custodian of the Spencer family estate, Althorp House.
Charles released this throwback picture with Princess Diana
The post comes shortly after it was revealed that BBC News religion editor, Martin Bashir, had stepped down from his role. He was the journalist behind Princess Diana's controversial Panorama interview.
It was also confirmed that Lord Dyson's investigation into how he secured the interview with Diana in the first place has now concluded. And Charles made his feelings pretty clear.
Last November, he demanded the BBC launch an inquiry after he claimed he was shown "false bank statements" by Martin to help encourage his sister to give an interview to the Panorama programme back in 1995.
Princess Diana and her brother Charles grew up in Althorp House
Elsewhere, Diana's brother Charles has very close links to the royal family; not only is he Prince William and Harry's maternal uncle, but he is also the godson of Her Majesty.
His father, John Spencer, 8th Earl Spencer, had worked as an equerry for King George VI and a young Queen Elizabeth. The Queen herself had been guest of honour at John's wedding to Frances Rocheg at Westminster Abbey on 1 June, 1954.
The Spencer family also rented out Park House on the monarch's Sandringham estate, and Prince Andrew and Prince Edward were frequent visitors to their home, often coming over to swim in their pool.
Make sure you never miss a ROYAL story! Sign up to our newsletter to get all of our celebrity, royal and lifestyle news delivered directly to your inbox.