Complex sentence: The Canadian Royal Family often stayed at Windfields when they visited Toronto; the last royals to stay there were Queen Elizabeth, the Queen Mother, in the summers of 1974 and 1981 and Prince Charles and Diana, Princess of Wales.
Split into simpler sentences: The Canadian Royal Family often stayed at Windfields when they visited Toronto. The last royals to stay there were Queen Elizabeth, the Queen Mother, in the summers of 1974 and 1981, and Prince Charles and Diana, Princess of Wales.