Complex sentence: Isabel Stillman Rockefeller (June 23,1902--March 23,1980), her father, Percy Avery Rockefeller, was one of the richest financiers and industrialists of his time.
Split into simpler sentences: Isabel Stillman Rockefeller (June 23,1902--March 23,1980) was a member of the Rockefeller family. Her father, Percy Avery Rockefeller, was one of the richest financiers and industrialists of his time.