Complex sentence: His parents believed that attending Whittier High School had caused Richard's older brother Harold to live a dissolute lifestyle before he fell ill of tuberculosis (he died of the disease in 1933), and so they sent Richard to the larger Fullerton Union High School.
Split into simpler sentences: His parents believed that attendance at Whittier High School caused Richard's older brother Harold to live a dissolute lifestyle before the older boy fell ill of tuberculosis (he died of the disease in 1933). Instead, they sent Richard to the larger Fullerton Union High School.