In a charter issued upon his accession, Thomas described Tvrtko II as his patruus, a Latin word that means "paternal uncle" if used as a noun, but "paternal uncle's son" if used as an adjective. The uncertainty is somewhat clarified by Ostoja's charters in which he referred to Tvrtko I, Tvrtko II's father, as his "late brother". The historian Dominik Mandić notes that, while cousins sometimes addressed each other as brothers, it would have been highly unusual for a person to describe his or her father as brother. Mandić thus concludes that Thomas was a patrilineal first cousin of Tvrtko II.[9]
