Judi said Princess Anne and Charles, who are both working royals, may have pulled together to support the Queen.
She added: “Senior royals do seem to have closed ranks around the Queen recently, creating an elite squad of Charles, Camilla and Anne, Kate and William and Edward and Sophie.”
Princess Anne is also thought to have a close relationship with Camilla, Duchess of Cornwall.
The two royal women are similar in age and have shown signs of friendship since Camilla married Prince Charles in 2005.
The close relationship between Princess Anne and Camilla could have also helped the brother and sister to become close, the expert added.