The Queen Consort’s younger sister Annabel Elliot will have a formal role during the coronation as a Lady in Attendance.
Camilla, 75, will be supported by Mrs Elliot, 74, and another Lady in Attendance, Fiona, 68, the Marchioness of Lansdowne, who was named as a Queen’s Companion last November.
The Queen Consort will be crowned alongside the King during the May 6 coronation service, and will also have four Pages of Honour who will perform roles alongside the Ladies in Attendance.
Mrs Elliot, an interior designer, is close to her sister and on rare occasions has taken part in royal events with her older sibling.
During an ITV documentary screened last year the pair even gave an insight into their family life when children, when a film crew accompanied them to Hall Place, the former Hampshire home of their grandparents.
It emerged in the show that Camilla buried her younger sister’s beloved teddy bear, known as “Tiddy Bar”, in the grounds and confessed only decades later.
Camilla said: “Yes, Tiddy Bar, he had a very happy resting ground,” and her sister joked: “I’ve not forgiven her, it still rankles to this day.”
Around 2,000 guests are expected to attend the coronation with King Willem-Alexander and Queen Maxima of the Netherlands among the first guests to confirm their attendance.
US President Joe Biden is not travelling to the UK for the coronation but First Lady Jill Biden will represent America.
The Duke of Sussex, who has a troubled relationship with Charles, will be part of the congregation to see his fathered crowned.
Guests have received detailed information about the coronation day from required clothing to toilet opening times.
Women are asked to wear day dresses, with hats and fascinators optional, while for men morning coats, lounge suits and uniforms are acceptable. Both sexes can wear national dress and decorations such as an insignia for an MBE or knighthood.
The Ministry of Defence has issued guidance for the three services, with men and women from the military required to wear their most formal uniforms.
The coronation service is expected to begin shortly after 11am but the majority of guests have been advised to arrive at a ticket and security checkpoint almost five hours ahead and be seated by 9am.
Later in the morning heads of state, Cabinet members, former prime ministers, foreign royalty and the royal family will enter Westminster Abbey ahead of the arrival of the King and Queen Consort with the service due to end at 1pm.