Complex sentence: While he was at Harvard, his fifth cousin Theodore "T. R." Roosevelt, Jr. (1858--1919) became President of the United States; his vigorous leadership style and reforming zeal made him Franklin's role model and hero.
Split into simpler sentences: While he was at Harvard, his fifth cousin Theodore "T.R." Roosevelt, Jr. (1858--1919) became President of the United States. Theodore's vigorous leadership style and reforming zeal made him Franklin's role model and hero.