Her older sister, Kate, will never be queen of England. She’ll always be the wife of the king. The same as Elisabeth II is the queen and her husband is not the king but the duke of Edinburgh.
Actually, Kate will become queen. Her technical title will be “queen consort” because she is not of royal blood, just as Her Majesty Queen Elizabeth The Queen Mother was. Since “queen” is a lower-ranking title than King, Kate will be crowned queen. Prince Philip cannot be king because he would outrank his wife, who is of royal blood.