Сигизмунд Кейстутович
Władysław I's relations with Lithuania remained ambiguous, especially after Prince Casimir, younger brother of King Władysław III became in the new Grand Duke in 1440. At that moment the Masovian rulers strongly supported Michael Žygimantaitis, the son of Sigismund Kęstutaitis, who was not only favorably accepted in the courts of Płock and Warsaw (who caused political agitation) but also married with Władysław I's youngest sister, Catherine.