As far as I know if you have the status HRH in the UK you dont curtsey to other HRH's-Kate will never be curtseying to Bea and Eug
There is I think a misunderstanding with regard to what happens when the "married in" are present but their other royal halves are not.It means that if they re entering a room the "blood royals" will go first.But the HRH's will only curtsey to the Queen and the Duke of Edinburgh
I will say immediately that I am not very knowlegdeable about curtseys and who should curtsey to whom.
But changes were made in the brf after Cam married in, as Anne did not want to curtsey to Cam (as she would have to as Cam is in rank above Anne), so changes were made that recognizes members "of the blood" above those who are married in. Kate does according to new rules have to curtsey to multiple people, depending though whether Bill is present or not. I think this "new" system is overly complicated and even though I'm far from being a Cam fan, I think Anne should have sucked it up. Or the brf does away with all the curtseying nonsense, like other royal houses did.