New rules for the BRF regarding order of precedence. If William is present at an event then the blood princesses curtsey to Kate. If she's alone then it is the other way around.Why would Princess Eugenie - a Princess by birth, be curtseying to Kate?
Apparently this was an issue going back to the days when Anne had to curtsey to her sisters-in-law Diana and Sarah due to their positions as Princess of Wales and Duchess of York.