She was the daughter of Michele Sbarra Morosini, Patrician of Venice. Her brother was Albertino Morosini.
In 1263 she married Stephen the Posthumous, Prince of Hungary in Venice where he was living in exile. His paternity was disputed by his brothers born from their father's earlier marriage and he was not recognized as heir presumptive to the Hungarian throne. Tomasina gave birth to a son, Andrew III, King of Hungary (ruled 1290-1301), who was named after his grandfather, Andrew II.
She supported her son's claim to the Hungarian throne. After the death of King Ladislaus IV, Prince Andrew succeeded to gain the kingdom and he was crowned King of Hungary in the name of Andrew III in 1290. He summoned his mother to Hungary and appointed her Princess of Slavonia.
Allegedly she was poisoned in 1300, shortly before her son's death on 14 January 1301 which was also rumoured to have been caused by poisoning. He was the last king of the House of Árpád.
Sources
Galambosi, Péter (2017). "Csák Ugrin, a hűséges tartományúr [Ugrin Csák, the Loyal Provincial Lord]". Fons (in Hungarian). 24 (3). Szentpétery Imre Történettudományi Alapítvány: 289–322. ISSN1217-8020.
Kristó, Gyula (1979). A feudális széttagolódás Magyarországon [Feudal Anarchy in Hungary] (in Hungarian). Akadémiai Kiadó. ISBN963-05-1595-4.
Wertner, Mór (1892). Az Árpádok családi története [Family History of the Árpáds] (in Hungarian). Szabó Ferencz N.-eleméri plébános & Pleitz Fer. Pál Könyvnyomdája.
Zsoldos, Attila (2003). "III. András". In Szovák, Kornél; Szentpéteri, József; Szakács, Margit (eds.). Szent István és III. András [Saint Stephen and Andrew III] (in Hungarian). Kossuth Kiadó. pp. 119–227. ISBN963-09-4461-8.
Zsoldos, Attila (2005a). Az Árpádok és asszonyaik. A királynéi intézmény az Árpádok korában [The Árpáds and their Women: The Institution of Queenship in the Era of the Árpáds] (in Hungarian). MTA Történettudományi Intézete. ISBN963 8312 98 X.
Zsoldos, Attila (2005b). "Ducissa Sclavonie [Duchess of Slavonia]". In Almási, Tibor; Draskóczy, István; Jancsó, Éva (eds.). Studia professoris–professor studiorum. Tanulmányok Érszegi Géza hatvanadik születésnapjára (in Hungarian). National Archives of Hungary. pp. 381–390. ISBN963-631-173-0.