Nick's Mistress(possible spoilers)

I haven't seen this yet.(I will be buying it in a few days, when I have the money,) but why was Nick's mistress a problem? It couldn't be just the fact that she was his mistress.Most mobsters, even though almost all of them are married, and have children, have mistresses. Did they feel she posed a threat somehow, and didn't fit in with the rest of the mob wives and mistresses? Did she know what Nick was doing? Do they try to get Nick to kill her? Also, the way Nick meets Ali, from the trailer, kind of reminds me of how I heard Whitey Bolger met one of his mistresses.(Or more like a common-law wife.) He met her at a diner where she worked. (I heard the Irish Mob is actually bigger then the Italian Mob in Boston.)