Have we ever seen either of the Wales duchesses curtsy to one of the Princesses of York?
They don't curtsy to people of the same rank (HRH by blood or marriage, whether with husbands or not). That notion, as best as I can determine, arose some years from one of the tabloids taking a change in the rules of precedence (who enters first, last, etc.) and turning it into curtsying to each other. It spread and gets repeated, but it is specious.
Kate has sometimes curtsied to the DofE, I assume as a sign of respect, but she doesn't have to.