Estefania is Jimmy's wife and the drug lord's daughter.
At some point, she married Jimmy yet had a flame with a man named Marco. In season 2, Estefania only speaks Potuguese and very little English and because of this nobody, except possibly Lip and Frank understood her, her English in season 3 is seems to be improved a lot.
She is introduced as the supposed girlfriend of Jimmy but it is later revealed they are married, due to her father forcing the arrangement on Jimmy.
Estefania did not think highly of Jimmy, openly kissing another man in front of her while disparaging the Gallagher family in Brazilian Portuguese. At the ending of Season 2, Marco abuses her, causing her to take up residence at the Gallagher's.
It seems things appear to be normal between Estefania and Marco because they are seen living together when Estefania's father comes to the States and quickly puts a bullet through Marco's head. Her father Nando is arguably one of the two main antagonists of season 3.