Cornelia is another one of the play's few "good guys"—though this doesn't work out very well for her. She's the mother of two corrupt children—Vittoria and Flamineo—and one nice boy named Marcello. Naturally, Marcello is the first to die—murdered by his own brother, Flamineo. This drives Cornelia insane with grief.
Earlier in the play, she overhears Brachiano, Vittoria, and Flamineo, plotting adultery and (in a veiled way) murder. When she confronts Flamineo, she asks him:
What! because we are poor
Shall we be vicious? (1.2)
For her, the answer is obvious: no, we shouldn't be vicious. But for Flamineo, it's very much an open question, and he ends up answering it with a resounding yes. He's a member of the "nice guys finish last" school of thought. But for Cornelia, there's nothing wrong with finishing last if you manage to keep your goodness and basic integrity intact.
In fact, when Cornelia laments Marcello's death, it reminds Flamineo of his own lost sense of compassion and goodness. It's creates a sense of missed possibility, as the play spirals down to its final moment of doom.