He was the third son of Stjepan Vukčić Kosača, Duke of Saint Sava, the most powerful noblemen in the Kingdom of Bosnia. Stjepan's half-siblings from his father's first marriage included Queen Katarina, wife of King Stjepan Tomaš, and Vladislav Hercegović, their father's successor. Stjepan's family belonged to the Bosnian Church, but were "shaky Christians" like most of their countrymen; his half-sister converted to Roman Catholicism upon marriage, while Stjepan himself adopted Islam and changed his name to Ahmed after moving to Constantinople in about 1473.[2][3] Hersekli Ahmed Pasha was a five-time Grand Vizier of the Ottoman Empire and Grand Admiral to the sultan, serving five times as Grand Vizier in the period from 1497 to 1515.