 On 6 September 1766, her mother married secondly, and in secret, Prince William Henry, Duke of Gloucester and Edinburgh, a member of the British Royal Family as the younger brother of King George III.It was this marriage, made without the King's consent, which had led to the passing of the Royal Marriages Act in 1772.Elizabeth acquired three half-siblings, Princess Sophia, Princess Carolina (1774–1775), and Prince William Frederick by her mother's second marriage to the Royal duke.