Prince William
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Yes.
Her mother in law is the late Diana, Princess of Wales who was mother to her husband, the Duke of Cambridge (Prince William.) Camilla was never a "mother in law" to the Duchesss of Cambridge as she is not Prince William's mother.
She could be HRHPrincess William, but she seems to go just by the DUCHESS OF CAMBRIDGE.
Windsor
Kate Middleton will become a princess when she marries Prince William. If William becomes king, Kate would then become queen. If Queen Elizabeth II abdicates the throne to Prince Harry however, Kate will remain a princess. This would seem unlikely considering William is in line to the throne before Harry.
No, Kate will be called Her Royal Highness Princess of Whales. That or longer. The royal family does not take the last name from Bride.
Windsor
If he dies without children, Kate will be the dowager (whatever her former title was). And then Harry and his line will succeed to the throne. However, if William dies before he marries Kate, then Kate gets no title at all.
William is 28 and Kate is 29
Prince William will marry Kate Middleton.