Kate Middleton could one day sit as Queen of England by virtue of her marriage to Prince William, the second in line to the throne. The Duchess of Cambridge, however, doesn't share her husband's rank for now as Queen Elizabeth ordered her to curtsy to blood princess like Princess Beatrice and Princess Eugenie when Prince William is not around.
Princess Beatrice and Princess Eugenie are the ninth and tenth in line to the throne thus they are considered low-ranking royals. Kate, aside from being Prince William's wife, is also the mother of the third in line to throne so she has seniority over most everyone else in the royal family.
However, Queen Elizabeth updated The Order of Precedence when Kate married into the family. According to reports, Kate was instructed to curtsy to the blood princess if she's going solo at royal family events. Aside from Princess Beatrice and Princess Eugenie, the other blood princesses are Queen Elizabeth's daughter Princess Anne and Queen Elizabeth's first cousin Princess Alexandra.
Princess Anne's daughter, Zara Phillips Tindall, is technically also a blood princess. Currently the 18th in line to the succession, Zara, who was born a few years before Prince William, doesn't have a royal title.
Zara's mother wanted her daughter and her son, Peter Phillips, to grow up normally, so she refused the Queen's offer to give her children any titles. So, if Zara is not a titled princess, Kate doesn't have to curtsy to her whenever Prince William is not around.
The Order of Precedence also dictated who should arrive at the event at a royal event first. In 2005, Queen Elizabeth also updated The Order of Precedence when Camilla, the Duchess of Cornwall, married Prince Charles. Thus, when Camilla attended a memorial service in 2006 without Prince Charles, she was forced to wait during a drizzling weather outside of Windsor until Princess Anne entered the venue.
To curtsy or to bend the knee is to show respect to a royal as a formal greeting, however, the family keep things more casual in these modern times. The public only sees them curtsy during formal royal events, except whenever Queen Elizabeth is around.
So, one day, Kate could curtsy in front of her husband in public and she might also curtsy to her son Prince George. These scenarios could only take place when the princes are crowned as the reigning monarch.
