King Władysław II, Euphemia's uncle, wanted to make a close bond between Kraków and Upper Silesia, and for this reason he was probably instrumental in her marriage with Bolesław I, Duke of Cieszyn.
After Bolesław I's death on 6 May 1431, Euphemia took the regency of the Duchy of Cieszyn on behalf of her minor sons, who inherited the lands as a co-rulers.
She continued her husband's policies supporting the towns and in 1438 co-decided, with her son Wenceslaus I, to give Cieszyn the right to mint coins.
With her consent, her sons were allowed to sell the Duchy of Siewierz to Zbigniew Oleśnicki, Bishop of Kraków, on 24 December 1443.
On 29 November 1442 the co-Dukes of Cieszyn made a formal division of their domains; however, the unity of the Duchy was maintained during Euphemia's lifetime.