Stephen Ostoja's illegitimate son Stephen Tomas Ostojic had succeeded his uncle Tvrtko II as King of Bosnia in 1443.
Steven Tomas is known to have had a wife Vojača, a commoner he had promised to marry.
After his enthronement, it had become clear that she was unsuited to be a queen.
Because of this, he had sought out Pope Eugene IV, who on May 29, 1445, absolved the marriage and declared Thomas to be the legitimate king of Bosnia.
Thomas had soon made peace with Stjepan Vukčić Kosača, the powerful noble ruling today's Herzegovina region.
The king is now searching for a new wife, and decides to marry Vukčić's daughter Katarina Kosača, which brings some political stability to the kingdom.
They are married on May 26, 1446 in Milodraža near Fojnica.
The couple have three children: Catherine, Sigismund, and another son about whom very little is known.