Complex sentence: Her mother, Beata Kościelecka, for many years tried to cancel marriage with Łukasz Górka, in the meantime Prince Siemion Olelkovycz Slutski died.
Split into simpler sentences: Her mother, Beata Kościelecka, for many years tried to cancel the marriage with Łukasz Górka. In the meantime, Prince Siemion Olelkovycz Slutski died.