In what year did Sigismund Casimir's mother die?
Cecilia Renata of Austria Archduchess Cecilia Renata of Austria (German: "Cäcilia Renata"; Polish: "Cecylia Renata"; 16 July 1611 – 24 March 1644) was Queen of Poland as consort to the Polish-Lithuanian Commonwealth's King Władysław IV Vasa.
Michael Žygimantaitis Michael Žygimantaitis (Lithuanian: "Mykolas Žygimantaitis" , Polish: "Michał Bolesław Zygmuntowicz" ; before 1406 – shortly before February 10, 1452 in Moscow) was the last male descendant of Kęstutis, Grand Duke of Lithuania. He supported his father Sigismund Kęstutaitis in power struggles with Švitrigaila. In 1435 he led his father's army to victory in the Battle of Pabaiskas. After his father's assassination in 1440, Michael unsuccessfully fought against Casimir IV Jagiellon for the title of the Grand Duke of Lithuania. At first Michael was supported by Samogitia, but in 1441 Casimir issued a privilege confirming Samogitia's semi-autonomous status and thus avoiding a civil war. Until 1447 he was supported by Dukes of Masovia. In 1448 Michael asked help from the Crimean Khanate. With their help he attacked and shortly took control of Kiev, Starodub, Novhorod-Siversky. Soon he was captured and transported to the Grand Duchy of Moscow where he died under unclear circumstances (possibly poisoned). He was buried in Vilnius Cathedral.