Complex sentence: When the 85-year-old Clark Griffith died in October 1955, his nephew Calvin and niece Thelma Griffith Haynes inherited 52 percent of the team's stock and majority control, with Calvin becoming the Senators' president and de facto general manager.
Split into simpler sentences: When the 85-year-old Clark Griffith died in October 1955, he owned 52 percent of the team's stock. His nephew Calvin and niece Thelma Griffith Haynes each inherited half of their uncle's shares and assumed control of the franchise, with Calvin becoming the Senators' president and "de facto" general manager.