premise: Sigismund Casimir, Crown Prince of Poland (Polish: "Zygmunt Kazimierz Waza" ), (1 April 1640 – 9 August 1647) - was the only legitimate son of King Władysław IV and his first wife Queen Cecilia Renata. He was named after his grandfather Sigismund III, and uncle John Casimir.
hypothesis: King Władysław IV and his first wife Queen Cecilia Renata had illegitimate children other than the Crown Prince.
reason: We are told that the Crown Prince was the fist legitimate son of King Władysław IV and his first wife Queen Cecilia Renata, eluding to their being illegitimate children as well. The AI can not know this from the text.