Complex sentence: Oney Judge remained a dower slave all her life, and legally her children also were dower slaves, property of the Custis estate, despite the fact that their father, Jack Staines, was a free man.
Split into simpler sentences: Oney Judge remained a "dower" slave all her life, and legally her children also were "dowers," property of the Custis Estate. This, despite the fact that their father, Jack Staines, was a free man.