Sorry, Dman (post 1748).
Who curties to whom?
The Queen altered the Order of Precedence in the Royal family twice in recent times: first, following the Prince of Wales' marriage and second, after William's.
The new Rules of Court make it clear that the Duchess of Cambridge, when she is not accompanied by Prince William, must curtsy to the “blood princesses”, the Princess Royal, Princess Alexandra, and the daughters of the Duke of York, Princesses Beatrice and Eugenie. Earlier, after Charles remarried, the Queen changed the Order of Precedence “on blood principles” so that neither Princess Anne nor Princess Alexandra, the granddaughter of George V, would have to curtsy to Camilla when her husband was not present. The Duchess of Cambridge, when William is with her, needs only to curtsy to the Prince of Wales, the Duchess of Cornwall, the Queen and the Duke of Edinburgh.
Although the etiquette may seem arcane, it is taken very seriously by the Royal family, whose members bow and curtsy to each other in public and in private. The Order of Precedence affects other aspects of royal protocol, such as who arrives first at an event. For example, the Duchess of Cornwall, in 2006, was forced to wait in the drizzle outside the Guards Chapel, Windsor, for the arrival of Princess Anne at a memorial service, because Charles had not accompanied her.
These rules seem faintly ridiculous to me but they're what Her Majesty decided, and, after all, it's her Court and her family.