Royal biographer Andrew Morton spoke with ET at Methodist Central Hall Westminster in England on Monday.
Despite their highly publicized rift, Prince Harry and Prince William have come together several times in the wake of their grandmother, Queen Elizabeth II's, death to honor the late monarch in a series of public and private ceremonies.
ET spoke with royal biographer Andrew Morton at Methodist Central Hall Westminster in England on Monday and discussed Harry and William's notorious rift, and how he feels their mother would have reacted.
"Well, the one thing that she would be really furious about is that they had this split," Morton began, explaining that he had previously discussed the boys' relationship with Diana herself. "She always said to me, not just once, but on several occasions, that Harry is William's backup in the nicest possible way, he's his wingman, he's the one who helps William in this difficult job that he will have in the future because it's a lonely job. And she always felt that Harry should be there as his backup, and I think everybody wants them to come back together again because together they are so much stronger."
On Sept. 8, Buckingham Palace announced that Her Majesty Queen Elizabeth II had "died peacefully" at 96 years old. Before the news of her death was made public, Prince William, Prince Harry and other immediate royal family members traveled to Balmoral Castle in Scotland to pay their final respects to the monarch.
Two days later, the estranged brothers and their wives, Meghan Markle and Kate Middleton, first reunited on Sept. 10 outside of Windsor Castle as they viewed floral tributes to the late queen.
ET learned, at the time, that William had reached out to Harry about an hour before the moment and asked if Harry and Meghan would like to join him and Kate. What's more, a source told ET that the Prince of Wales thought it was an important show of unity at an incredibly difficult time for the family.
With regard to the solidarity the public has seen between Prince Harry and Prince William since the queen's death and throughout the several events and ceremonies leading up to Monday's funeral, Morton is impressed by the royal unanimity.
"Well, they've gone through this week or past few days out of respect and love for the queen and they behaved impeccably. Will they come back together? It's down to those two," Morton noted.
Looking to the future, Morton does not predict that Prince Harry and Meghan will return to call Britain home anytime soon.
"You know, Meghan and Harry are happy and have a life," Morton said. "As Prince Charles said in his opening statement, in California, they're the king and queen of California, their daughter is now an American princess, the first American princess since Lily Van, but they go to local schools. I don't see them rushing back to Britain, apart from the occasional visit to see their charities," he explained.