Complex sentence: Hobbes was employed by another branch of the Cavendish family (Devonshire Cavendishes); his association with Welbeck started in 1631, when he was tutor to a different Earl of the same name, William Cavendish, 3rd Earl of Devonshire.
Split into simpler sentences: Hobbes was employed by another branch of the Cavendish family (Devonshire Cavendishes). It was certainly by 1631, when he was tutor to a different Earl of the same name, William Cavendish, 3rd Earl of Devonshire.