Why Prince William Hasn't Been Able to 'Forgive' Prince Harry for Leaving the Royal Family (Exclusive)

Royal Expert Katie Nicholl opened up to ET about the royal strife, detailed in her new book, 'The New Royals.'

Can there ever be a return of brotherly love in the English Monarchy? Prince William's falling out with his brother, Prince Harry, has left a hole in the Royal Family, and many are wondering if there will ever be a reconciliation.

Royal expert Katie Nicholl recently sat down with ET's Rachel Smith and opened up about William's feelings toward Harry and how the newly crowned King Charles III might try to mend their fences, which she wrote about in her new book, The New Royals: Queen Elizabeth's Legacy and the Future of the Crown.

"Everyone wants to see a repair in the rift because it doesn't do anyone any good to have this rift in the heart of the House of Windsor," Nicholl shared. I spoke to aides, past and present -- including one senior aid who worked with the brothers for a good 10 years, who knows them both and still speaks to them both. And he was surprisingly confident that there could be a rapprochement."

"But that will be some time down the line," Nicholl added. "And that comes down to the fact that William simply cannot forgive Harry. Not just for leaving, but for how he left."

According to Nicholl, when Harry and his wife, Meghan Markle, decided to leave the U.K. and move to England, while also formally stepping away from their roles in the Royal Family, it drastically altered the plans that had been first laid out years ago by senior royals, including Charles and the late Queen Elizabeth II.

"Harry, being the spare, was always going to be [William's] sort of deputy and that was very much part of Charles' vision, you know? The image that we always go back to is the image we saw a couple of days ago -- of the king, Camilla, William, and Kate -- Charles has always envisioned Harry and Meghan being a part of that, as had the queen."

"It's why she gave them those plum roles as ambassadors in the commonwealth," she added. "The idea was never for Charles to have the cousins and the extended family up on that balcony with him...  it was always going to be inclusive of Harry and Meghan."

"I think the Royal Family recognized that they could be very good brand ambassadors, they were real assets, but it ultimately wasn't what they wanted," she shared. "They didn't they didn't fit into the hierarchy the way that it was hoped, and we now know all the power struggles that went on behind the scenes."

According to Nicholl, Harry's decision to step away from his role as a royal had "a really immediate and very real impact" on both William and King Charles. In the case of Charles, the rift is particularly difficult as it has been "overshadowing the start of his reign."

"I think it's very difficult for the king to force a reconciliation on anyone," Nicholl shared. "It's either going to happen or it’s not... it is his genuine hope that he can smooth things out. I mean, not least because it doesn't look good for the monarchy to have a rift at the center of it, but as father, he wants to sort things out. As a grandfather, he wants to have a relationship with his grandchildren."

That being said, if there can be no healing, or if Harry and Megan don't heed the Royal Family's wishes when it comes to loyalty and secrecy, some have suggested that Charles may take away the royal titles for Harry and Meghan's children -- 3-year-old son Archie and 1-year-old daughter Lilibet.

"If Charles gives them those titles, and I think it's a big if, that comes with a cost. That's going to come with an expectation of loyalty and trust," Nicholl shared. "If he doesn't give them the titles, which are lawfully theirs ... well is that just going to be the final nail in the coffin for the relationship? These are the really big issues that Charles is having to contend with."

Nicholl's revealing new book on the modern monarchyThe New Royals: Queen Elizabeth's Legacy and the Future of the Crown, is out now.