The 62-year-old has spoken about Diana in a series of interviews accompanying his newly released memoir, Swan Song: Diana, My Sister, as the 30th anniversary of her death approaches in 2027.
Speaking to People at Althorp, the Spencer family estate in Northamptonshire where Diana is buried, Charles said he had helped his sister during her final years but still questions whether he should have intervened more.
"Could I have done more to help her with her public life? I probably could," he said.
"Towards the end of her life, I did help her a bit. I had had some run-ins with the media. I helped her win a big libel case against a British newspaper. I would fight some battles for her quietly."
Diana was 36 when she died following a car crash in Paris in August 1997, a year after her divorce from the then-Prince Charles.
Charles Spencer's new book has returned the relationship between Diana, her brother and the royal family to intense public scrutiny, with Charles making a series of claims about events surrounding her marriage and death that have been disputed by Buckingham Palace.
Charles also used his People cover interview to challenge a longstanding claim that he refused to provide Diana with a home at Althorp after her divorce.
He said: "One of the things people are throwing at me totally wrongly is that I wouldn't give her a home."
Charles said Diana had asked whether she and her sons, Prince William and Prince Harry, could live at Garden House on the Althorp estate, where their sister Lady Jane Fellowes had previously lived with her family.
He said: "When she was divorced, she did ask me for the Garden House at the back of the park. Diana had loved it because our sister, Jane, lived there with her children, and that was perfect for them."
Charles said local police assessed the property and concluded protecting Diana and her sons there would not be practical.
"The local police went around there with me and they said, they just couldn't protect William and Harry," he said.
"She could not have lived at the Garden House. It was not a viable option."
Diana instead continued living at Kensington Palace following the end of her marriage.
The publication of Swan Song has become the latest controversy surrounding Diana's legacy after Charles alleged the then-Prince Charles told him during an argument over funeral arrangements after Diana's death: "Rest assured, we'll forget her soon enough."
