Who married Casimir II the Just?

Casimir II the Just (Polish: Kazimierz II Sprawiedliwy; 28 October 1138 – 5 May 1194) was a Lesser Polish Duke of Wiślica from 1166 to 1173, and of Sandomierz after 1173. He became ruler over the Polish Seniorate Province at Kraków and thereby High Duke of Poland in 1177; a position he held until his death, though interrupted once by his elder brother and predecessor Mieszko III. In 1186 Casimir also inherited the Duchy of Masovia from his nephew Leszek, becoming the progenitor of the Masovian branch of the royal Piast dynasty, and great-grandfather of the later Polish king Władysław I the Elbow-high. The honorific title "the Just" was not contemporary and first appeared in the 16th century.

Helena of Znojmo (Czech: Helena Znojemská; Polish: Helena znojemska; c. 1141–1202/1206), was a Bohemian princess, a member of the Přemyslid dynasty. She was the daughter of Duke Conrad II of Znojmo and his Serbian wife Maria (daughter of Uroš I). Helena was probably named after her maternal aunt, Queen Helena of Hungary, wife of King Béla II.

Born as princess of the Znojmo Appanage (named after its centre, the town of Znojmo in southern Moravia), she later became by marriage Duchess of Sandomierz (1173-1194), Grand Duchess of Kraków and the Seniorate Province (1177-1194), Duchess of Masovia (1186–1194), and Duchess-regent of Kraków and the Seniorate Province, Sandomierz, and Masovia on behalf of her minor sons from 1194–1198, then the latter two duchies plus Kuyavia until 1199/1200.