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Who knows - but unless they change the law to allow for gender neutral inheritance - that girl could be replaced by a younger son so no they wouldn't create the girl Princess of Wales in their own right.
Currently William and Harry are HRH Prince William of Wales and HRH Prince Henry of Wales while their father is HRH The Prince Charles.
Remember, however, Princess Charlotte who was formally Princess Charlotte of Wales. Had she lived, her uncles wouldn't have been forced into legal marriages and the world wouldn't have had Queen Victoria!
Victoria was never considered Princess of Wales because she wasn't the child of a reigning monarch and her uncle preceded her.
As for the present monarch, the Queen Mother was only 36 when her husband ascended the throne and for some years it was thought that a male heir was not impossible. Later, the present Queen wasn't made Princess of Wales because of possible complications when and if she married, i.e. what to call her future husband, and because it was thought that it would downgrade her status as a Princess of the UK in her own right. Letters Patent could have altered this situation, but it's believed that hidebound courtiers prevailed over the King and tradition was preserved.
Interestingly, although he never invested her as such, Henry VIII allowed his daughter Mary to sign herself Princess of Wales and to use the formal seal of Wales for her papers and correspondence, at least until the birth of Edward.
Given today's social conditions and expectations, if William's first child is a daughter, we can expect a renewed and deafening demand that the law be altered in the UK, as it has been elsewhere.