Princess Diana ‘always expected’ Prince Harry to be ‘wingman’ and ‘not a hitman’ to his elder brother, Prince William, the late royal’s biographer has said.The relationship between the two brothers has been regarded as strained since the Duke of Sussex, 38, announced he and his wife Meghan Markle were stepping back from royal life back in 2020.Kensington Palace, along with Buckingham Palace, is awaiting the publication of Prince Harry’s memoir, which has been touted as ‘unflinching’.It is thought the Firm will not receive advance sight of the book, which will be published on January 10, 2023.But as speculation over the contents of the ‘wholly truthful’ memoir continues, the late Princess Diana’s biographer, Andrew Morton, has said Harry and William’s mother would have wanted her younger son to support her eldest, rather than launch an attack.Mr Morton told TIME magazine: ‘Diana always expected Harry to be a wingman to William, not a hitman, so it will be interesting to see how the book is received inside the Royal Family.’ The brothers reportedly have a fraught relationship (Image: Getty) Prince Harry returned to the UK for the unveiling of a statue of Princess Diana (Image: Getty)Adding he will ‘be reading’ the memoir once its contents is made public, Mr Morton argued the Duke of Sussex ‘has every right’ to outline his experiences within the royal institution.Drawing parallels with the memoir released by the Duke of Windsor, or Edward VIII, who abdicated the throne in 1936, Mr Morton argued the monarchy ‘had survived many things, and it will certainly survive Spare’.Last month, publishers Penguin Random House confirmed the release date for the memoir after it was previously ‘tentatively’ set for late 2022.Along with a new website dedicated to the book, the name of the memoir, Spare, was also announced.READ MORE: Harry and Meghan could be ‘cast into wilderness’ by royals over book The brothers reunited with their wives after the death of Queen Elizabeth II (Image: Getty)The title is a reference to the Duke of Sussex’s position as the second-born child after his elder brother, Prince William, who is the heir to the throne.Promotional material said the memoir will take “readers immediately back to one of the most searing images of the 20th century: two young boys, two princes, walking behind their mother’s coffin as the world watched in sorrow-and horror.’The images of Prince Harry and Prince William at Princess Diana’s funeral in September 1997 were viewed across the globe.Prince Harry was 12 years old at the time, and Prince William was 15.DON’T MISS: Meghan accused of ‘showing off’ as new crown cypher emerges on letter [LIVE] Meghan Markle top choice for President – but would get rude awakening [REPORT] POLL: Was King Charles right to ban Prince Andrew from royal return? [VOTE] The two brothers were 12 and 15 years old when Diana died in Paris (Image: Getty)The publishers described the book as a “landmark” work.It will be “full of insight, revelation, self-examination, and hard-won wisdom about the eternal power of love over grief’, the publisher added.The book will be available in 16 languages, as well as in audiobook format.The Duke has earmarked $1.5m of the profits from the book to Sentebale, a charity established by Prince Harry and Prince Seeiso of Lesotho in 2006.A further £300,000 will go to WellChild, a charity of which the Prince has been a patron for 15 years.Penguin Random House’s chief executive, Markus Dohle, hailed the book as sharing a “remarkably moving personal journey from trauma to healing”.