Complex sentence: Her father was a life member of the Massachusetts Anti-Slavery Society; and her older brother Charles Lenox Remond was the American Anti-Slavery Society's first black lecturer and the nation's leading black abolitionist until Frederick Douglass appeared on the scene in 1842.
Split into simpler sentences: Her father was a life member of the Massachusetts Anti-Slavery Society. Her older brother Charles Lenox Remond was the American Anti-Slavery Society's first black lecturer and the nation's leading black abolitionist until Frederick Douglass became prominent in 1842.