Complex sentence: He married Clementia of Burgundy, sister of Pope Callistus II. They had three children, but only the oldest survived to adulthood: he succeeded Robert as Baldwin VII of Flanders.
Split into simpler sentences: He married Clementia of Burgundy, sister of Pope Callistus II. They had three children, but only the oldest survived to adulthood. He succeeded Robert as Baldwin VII of Flanders.