Along with The King and Queen Camilla, Prince William and Princess Kate, who share a close bond with the Edinburghs, are said to be joining the festivities. The Princess of Wales and Duchess of Edinburgh are said to have a particularly strong relationship, with Kate viewing Sophie as a sort of "royal sister."

As a source told VF, the Duchess of Edinburgh, a new family favorite, "prefers to keep the attention on her causes and charities rather than on herself."

Kate Middleton found a mentor in Sophie, the Duchess of Edinburgh when she was first welcomed into the royal fold, but their warm working relationship has developed into an even closer sisterly bond amid Kate’s cancer battle.