Complex sentence: Oney was a "dower" slave, owned by the estate of Martha Washington's first husband, and therefore not one of the 124 Mount Vernon slaves freed by George Washington's will following his 1799 death.
Split into simpler sentences: Oney was a "dower" slave, owned by the estate of Martha Washington's first husband. Therefore, she was not one of the 124 Mount Vernon slaves freed by George Washington's will following his 1799 death.