Downer alert: Pete dumps Maggie (no surprise there), and Maggie's wrathful lush of a mother tosses her out on the street. For both Jimmie and Mary, a misguided sense of pride and ridiculously irrelevant ideas about morals lead them to cruelly reject Maggie. Thanks for nothing, family.
With nowhere to go, Maggie hits the streets, where it is suggested (but not directly stated) that she becomes "A girl of the painted cohorts" (17.4). And then she dies. Her cause of death has been a subject of some debate. Does she throw herself into the East River? Is she murdered by "a ragged being with shifting, bloodshot eyes and grimy hands" (17.17)? It's not entirely clear, but we do know she is dead and that, given that this is a work of Naturalism, her grim demise has always been inevitable.
And as for the big fit her mother throws when Jimmie flatly relays the news? Well, she's a giant hypocrite. While we've known as much all along, what matters most in terms of the ending is that someone who's nasty and terrible and amoral survives while Maggie dies.