I think the same, but no one talked about other man in her life. That means that so far there are 0 evidence of Antonio being from another father. Even Gracie called him brother, the same girl that said to Isabel that she is the "real" family and Isabel isnt. So, Ill stick (for the moment) with Lucan being father of them three. Its really hard to tell, because Gracie never talked about Lucan either, but Cordia doesnt look like a woman who were able to be with more than one husband (at least thats what a feel).