Both Camilla and Kate take their husband's precedence when they are with them. It's because their place and rank is derived strictly from their marriage to them and not by anything else. In case of Princess Anne and other royal princesses by birth, their rights, rank and place in the order of precedence come from their birth, just like the husbands' of Camilla and Kate. Theoretically, Princess Eugenie married to her cousin Prince William (well, it would not be unusual in this family
would outrank her elder sister because of the rank held by her husband. But, according to the rules of Private Precedence or without her husband in attendance, she should place herself after Beatrice.
Camilla and Kate could be even the Byzantine Empress and the Empress of the Holy Roman Empire in the future but it really does not matter today. Their husbands are who they are now and they are their present wives and have rank and precedence according to that status. Period.