Commenting on such outings together, body language expert Judi James highlighted a change between the royals.
She claimed their body language would suggest a close relationship between the in-laws, however their friendship was not always as obvious.
When Camilla married Prince Charles, Judi suggested Princess Anne was “slow to thaw”.
“Given their past histories, with Anne having dated Camilla’s ex-husband, the relationship between Anne and Camilla might have been difficult,” the expert explained.
“History like that would normally create a sense of embarrassment or discomfort from someone in Anne’s position.
“But [in one photo] she looked as though she might be ignoring or cutting Camilla rather than avoiding her, suggesting she is taking a slow-thaw route to her sister-in-law.
“But now, the two women seem very tolerant of one another to the point of often looking like old friends.”
Princess Anne is thought to have briefly dated Camilla’s first husband Andrew Parker Bowles, many years before he dated Camilla.
However, sharing an ex does not seem to have damaged the relationship between the royals.
Despite appearing to take a “slow-thaw” approach to their relationship, Judi claimed it was not long before they showed signs of bonding.
The expert continued: “Camilla and Anne’s most frequent bouts of body language together suggest they have harvested all the many things they have in common to relax into the role of friends.”
She continued to say signs of their growing friendship were most obvious from 2007, around two years after Camilla joined the family.
Both women are of a similar age, have similar roles within the Royal Family and are both mothers-of-two. Such similarities may have helped them bond.
Judi continued: “In 2007, Camilla’s popularity with the public was rising and she was proving her worth as a member of the Firm via some tours abroad with Charles.
“In one photo, sitting in the carriage with Anne, Camilla wears a reasonably relaxed-looking smile.
“It’s the pose of the two women laughing together in the line-up that really seems to signal not just a thawing of the frost between them but a growing sense of close friendship between two women who have so much in common.”