Complex sentence: Both sons went to Eton, the elder Nicholas Fraser, and the younger, the art dealer Robert Fraser, known as "Groovy Bob" because he was so well-connected to the leading figures in Swinging London in the 1960s.
Split into simpler sentences: Both sons went to Eton: the elder Nicholas Fraser, and the younger, Robert Fraser,. As an adult, Robert became known as "Groovy Bob" because he was so well-connected to the leading figures in Swinging London in the 1960s.