On 12 December 1385, a few months after the Union of Krewo, Siemowit IV, Duke of Masovia, reached a compromise with Queen Jadwiga of Poland and her intended consort, King Jogaila (Władysław II Jagiełło), the brother of Alexandra.

Siemowit IV agreed to cease his rival claims to the Kingdom of Poland, pay homage to Jadwiga and Jogaila, and to assume the position of a hereditary vassal to the Polish crown in exchange for 10,000 Prague groschen and a fief (the Duchy of Belz).

[4] Alexandra was frequently pregnant during her union with Siemowit, giving birth to 13 children- five sons and eight daughters- in about 25 years.

Grandchildren of Alexandra and Siemowit IV included Frederick III, Holy Roman Emperor, Przemyslaus II, Duke of Cieszyn, Sophie of Pomerania, Duchess of Pomerania and Dorothy Garai.

Kraków: Polska Akademia Umiejętności – Skład Główny w Księgarniach Gebethnera i Wolffa, 1935, p. 66–67