Gothic tales - built on the flimsiest premises

It's as if "A Reliable Wife" is struggling to find an equipoise, some measure to goodness and nobility to serve as a counterweight against the unrelenting evil and bitterness of all of the central characters. Three characters essentially - Truitt, Catherine Land, Antonio - each obsessed with vengeance, tormented by inner demons, victims of horrible childhoods, pathological. But in part 3 of the book (or just before), a new character emerges - Catherine's sister, Alice - who is as bitter, lost, and evil as the others, and the book becomes engorged, supersaturated with gall, it can absorb no more of the same type of character, and Catherine, somewhat inexplicably, softens, becomes kinder. Truly, it's been hard to accept some of the elements of her character - would a woman raised on the streets as a hooker really devote so much time so self-improvement in municipal libraries, so much that she could create an credible persona as a daughter of missionaries? But now she becomes the great caretaker of her younger sister, we learn in one of the novel's expository chapters of her lifelong devotion to Alice. But Alice spitefully turns against Catherine, refuses her help (apparently), and Catherine decides she cannot carry through with the plan to kill her husband, Truitt. Why the change, the softening? Does she realize that in fact she's got all that she wanted - wealth, security, comfort - in marrying Truitt? Or is she, against all evidence, a good person, resisting the evil around her? Gothic tales such as this are often built on the flimsiest basis of coincidence (the actual plot mechanism that sets the story in motion is dependent on so many improbabilities that it's not worth even troubling about - just accept it) and on sudden redemptions, revelations. Let's accept, then, that Catherine's personality does change. What will become of her? Is she strong enough to prevail? Is her life with Truitt worth saving?