Complex sentence: His eldest son, the seventh Earl, died at a young age and was succeeded by his younger brother, the eighth Earl, who was killed in a road accident in Doncaster while on active service in the Second World War.
Split into simpler sentences: His eldest son, the seventh Earl, died at a young age and was succeeded by his younger brother, the eighth Earl. He was killed in a road accident while on active service in the Second World War.