Complex sentence: Her maternal great aunt, Gryzelda Dominika Zebrzydowska, was the prioress there until she died of the bubonic plague, at which point Stanislawka was placed in the guardianship of her maternal grandfather.
Split into simpler sentences: Her maternal great aunt, Gryzelda Dominika Zebrzydowska, was the prioress. Following her death of the bubonic plague, Stanislawka was placed in the guardianship of her maternal grandfather.