But what is the relationship like between Camilla and Sophie in the modern-day Royal Family?
Judi James, a body language expert, spoke exclusively to Express.co.uk about the relationship between the two wives of that royal generation.
Judi asked: “Could they be suffering from ‘First Wives Club’ Syndrome?
“Despite being sisters-in-law there are few, if any, body language moments between Sophie and Camilla that would suggest anything warmer than a polite and even rather formal relationship.
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“There is no mirroring between them to hint at natural empathy or like-minded thinking.
“Also, there is nothing to define similar or complementary personalities,” the expert claimed.
Judi expanded on Sophie’s warm relationship with other members of the Royal Family.
She suggested: “Sophie tends to be very open in terms of her signals of friendship.
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“She is fun and active warmth at all levels of the royal firm, from the Queen herself downward.
“Her relationship with Kate looks especially like one of spontaneous friendship and they will often be seen chatting and engaged as though genuinely enjoying one another’s company.
“But with Camilla, there seems to be an air of polite detachment.
“It is as though the two women will happily talk and acknowledge one another in public but without ramping up their rituals and displays to hint at stronger bonds,” Judi added.
Sophie and Camilla rarely attend royal engagements together unless the event involves several royals.
For example, in recent years they have only been photographed together attending Trooping the Colour or the annual Festival of Remembrance.
These are the only times the two are photographed together, suggesting a “formal relationship” as Judi said.
Camilla is 74 while Sophie is 56 so the two royal ladies are from completely different generations.
Sophie is raising an 18-year-old and a 14-year-old, Lady Louise Windsor and James, Viscount Severn.
Meanwhile, Camilla already has 10 grandchildren of her own.
She has five grandchildren from her two children, Laura Lopes and Tom Parker Bowles.
However, Camilla is also step-grandmother to Prince William and Prince Harry’s children, Prince George, Princess Charlotte, Prince Louis, Archie Mountbatten-Windsor and Lilibet Mountbatten Windsor.