Complex sentence: In 1825, Margaret Horniblow died and willed the twelve-year-old Harriet to Horniblow's five-year-old niece, whose father, Dr. James Norcom, became her "de facto" master.
Split into simpler sentences: In 1825, Margaret Horniblow died and willed the twelve-year-old Harriet to Horniblow's five-year-old niece. The girl's father, Dr. James Norcom, became Harriet's "de facto" master.