wrongful marriage?

im from NJ

and im worried for my sister her marriage to this guy smelled funny sense day 1 she told me he came from another country and over the years one by one his family members were come to the USA then now she is fighting with him because lack of attention he gives her, which really means his going to NY prob, cheating on her she tells me that he calls his wife at the other country, i ask my sister "wait he has a wife over there??" she says yes! now im thinking and told her this, hes trying to take my sis for every penny and only married her for the papers so he wont get deported and bring his other wife over to USA along with 7 kids of that women! on top he had a kid with my sis who's in the middle of all this, my question is was my sisters marriage legal ? sense hes still married at his country isn't that fraud?? and does my sis have a case for child to stay with her?