They aren't required to curtsey if they did it earlier in the day in private.
Like I said...
It's rare that Kate can be perceived as better behaved, but the York girls are the exception to that. They almost always look like they are giving the middle finger via dress, grooming, comportment, or behavior. So, when placed next to them, she comes off just about as close to pulled together as she ever will. (And I'm not a Kate fan...).