Complex sentence: He married Margaret "Peggy" McGrath (died 1996) on September 7,1940; (September 28,1915--March 26,1996), she was the daughter of a partner in a prominent Wall Street law firm.
Split into simpler sentences: He married Margaret "Peggy" McGrath (September 28,1915--March 26,1996) on September 7,1940. She was the daughter of a partner in a prominent Wall Street law firm.