Beatrice and Eugenie may not be used to plug the gap of the Royal Family due to fears they "might leak something," according to Angela Levin.
The Royal biographer has revealed who she believes could step up to take on royal duties as King Charles and Princess Kate begin their recovery from surgery.
King Charles left hospital earlier this week after undergoing a corrective procedure for a benign enlarged prostate.
Meanwhile, the Princess of Wales is recovering at home after spending 13 days at the London Clinic Hospital.
Angela Levin revealed who she thinks could help out the senior royals
GB News
Princess Kate had a successful planned abdominal procedure and is not expected to return to royal duties until after Easter.
Prince William has postponed planned royal engagements while he looks after his wife and their children, Prince George, Princess Charlotte, and Prince Louis
Speaking to GBN America about who the King may appoint to take over royal duties, Levin said: "He didn't know he was going to be unwell or that Catherine would fall ill because she eats well and all that sort of thing.
"So, I think he didn't have that in mind. It's very difficult to choose who he would have.
Eugenie and Beatrice are "close friends" with Prince Harry
PA"He's got his sister Princess Anne, who's always there, works incredibly hard and only does slightly fewer engagements than him. He's got the Duke and Duchess of Scotland, his youngest brother.
"And that's been terrific as well because they just get on with things, Sophie and Edward.
"Then you come to whether he should have his other brother's children and Prince Andrew's daughters, and that's very difficult as they are quite friendly with Harry.
"They would maybe leak something and it might be awkward because of the problems that Prince Andrew is still having."
Levin said that Zara Tindall and Mike Tindall "fit in beautifully" with the royals
PAShe added: "My view is that he could have Zara, who is the daughter of Princess Anne.
"She and her husband Mike, who also comes from an ordinary background, fit in beautifully with the Royal Family.
"They could do it very well. They're very popular, they're very nice, they're jolly, they're honourable, they're loyal to all the Royal Family.
"So he could do that. Maybe he will, but I think he's trying to save the country money. It might not be worthwhile."