Complex sentence: Her grandson was Cosimo I de'Medici, he became Grand Duke of Tuscany, his son was Francesco I de' Medici who succeeded his father in 1574, among Francesco's children were Eleonora de' Medici and Marie de' Medici.
Split into simpler sentences: Her grandson was Cosimo I de'Medici, who became Grand Duke of Tuscany, his son was Francesco I de' Medici who succeeded his father in 1574. Among Francesco's children were Eleonora de' Medici and Marie de' Medici.