Complex sentence: With her children, she was under the lifetime control of his widow, Martha Custis Washington, who remarried; after her death, Betty and her children would revert to the Custis estate for disposition.
Split into simpler sentences: With her children, she was under the lifetime control of his widow, Martha Custis, who had married Washington. After Martha's death, Betty and her children would revert to the Custis estate for disposition.