Complex sentence: Darst's mother, Doris Jeanne Gissy, was, at 14, the youngest person on the cover of Sports Illustrated in August, 1956 for horse riding along with her sister Ruth Gissy.
Split into simpler sentences: Darst's mother, Doris Jeanne Gissy, was, at age 14, the youngest person to appear on the cover of "Sports Illustrated". Gissy appeared on the magazine's cover in August 1956 for horse riding along with her sister Ruth Gissy.