Complex sentence: Her paternal grandparents were James Hamilton, 1st Earl of Arran and Lady Janet Beaton, and her maternal grandparents were James Douglas, 3rd Earl of Morton and Catherine Stewart, illegitimate daughter of King James IV of Scotland by his mistress Marion Boyd.
Split into simpler sentences: Her paternal grandparents were James Hamilton, 1st Earl of Arran and Lady Janet Beaton. Her maternal grandparents were James Douglas, 3rd Earl of Morton and Catherine Stewart, illegitimate daughter of King James IV of Scotland by his mistress Marion Boyd.