Complex sentence: Sir Allan MacNab's second daughter, Sophia Mary, married William Keppel, 7th Earl of Albemarle who are the ancestors of Camilla, The Duchess of Cornwall.
Split into simpler sentences: Sir Allan MacNab's second daughter, Sophia Mary, married William Keppel, 7th Earl of Albemarle. Sophia Macnab and William Keppel are the ancestors of Camilla, The Duchess of Cornwall.