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.
Treaty of Vilnius (1559) The Treaty of Vilnius or Vilna was concluded on 31 August 1559, during the Livonian War, between the Livonian Order and the Polish-Lithuanian Commonwealth at Vilnius (Vilna, Wilna, Wilno). Gotthard von Kettler put the lands of the order under protection of Polish king and Grand Duke of Lithuania, Sigismund II Augustus. The alliance was intended to neutralize the imminent threat of annexation of the order's lands by Russia, yet despite earning military support from Polish-Lithuanian chancellor Mikołaj "the Black" Radziwiłł, Kettler was unable to prevent the occupation of most of Livonia by Russian forces. Thus, Kettler concluded a second treaty with Poland-Lithuania on 28 November 1561, transferring the remnants of the Teutonic Order State to the Polish Crown and the Grand Duchy of Lithuania, himself becoming a vassal of Sigismund II Augustus.