Princess Beatrice and Princess Eugenie have been tipped to step up to fill in the "major absence" of senior royals.
The Firm has pulled together in recent weeks after King Charles III and Princess Kate announced they were both battling cancer.
Buckingham Palace was expected to look to slim down before the diagnoses, with Megxit creating instability.
Royal expert Phil Dampier said: "I know that Bea and Eugenie are willing to do more and would happily help out. They just need to be given the green light."
Princess Eugenie and Princess Beatrice
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He added: "With the King and Princess of Wales battling cancer and with Harry and Meghan and Prince Andrew off the scene, the slimmed-down monarchy is looking a bit thin.
"The late Queen and Prince Philip had over 1,500 patronages between them and most of them have not now got a royal patron."
Dampier also stressed there could be a place for Prince Edward, Countess Sophie, Lady Louise and Viscount James.
Beatrice and Eugenie are fully styled and titled members of the Royal Family.
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King Charles
GettyHowever, the sisters have never been asked to become senior working royals.
Beatrice lives in a four-bedroom apartment in St James’s Palace alongside her husband Edoardo Mapelli Mozzi and daughter Siennea.
St James’s Palace is the most senior royal palace in London.
The original redbrick building was built between 1531 and 1536 by Henry VIII.
Beatrice and Eugenie posing at the Vogue World eventInstagram / Princess EugenieDespite Dampier’s comments, King Charles appeared to snub Princess Eugenie.
Eugenie, unlike Beatrice, is not a Counsellor of State.
Counsellors of State are usually the Sovereign's spouse and the next four people in the line of succession over the age of 21.
Princess Anne and Prince Edward were also promoted under the Counsellors of State Act 2022 following the fallout of Megxit and Prince Andrew’s scandal.
King Charles
GettyRoyal historian Marlene Koenig told GB News: "When Charles became King, he was no longer a Counsellor of State.
"So, according to the Regency Law, Princess Beatrice moved up to that spot.
"Technically Prince Harry should not be on it because he's not domiciled in the UK. But no, Charles does not want to upset the Sussexes.
"Prince Andrew, who is allegedly accused of things, hasn't been removed either."