Complex sentence: The best man at his wedding was his cousin William Harriman of New York, Henry Flagler attended with his wife and he also invited a large number of African-Americans which went against the social customs of the time.
Split into simpler sentences: The best man at his wedding was his cousin William Harriman of New York, and Henry Flagler attended with his wife. Anderson also invited a large number of African-Americans which went against the social customs of the time.