First, Mattie is not particularly good in her role as “helper” to Zeena. She doesn’t enjoy housework, and her mind tends to wander (Wharton writes that “Mattie had no natural turn for housekeeping, and her training had done nothing to remedy the defect. She was quick to learn, but forgetful and dreamy, and not disposed to take the matter seriously.”) Second, Zeena’s “sickliness,” and her desire for live-in help, is clearly an attempt on her part to exert power over Ethan and the household; Mattie is doubly unsuited for this plan, since she is not much of a help and actually becomes competition for Ethan’s affections, something Zeena sees with mounting jealousy. This jealously finds expression in Zeena’s observation that Ethan is now shaving every morning (since the arrival of Mattie), and in locking them out of the house one night when Ethan is walking Mattie back from town.
On a deeper level, Zeena hates Mattie for the same reasons Ethan is drawn to her. While Ethan sees in Mattie someone who could be taught the intellectual musings he usually keeps to himself (she “had an eye to see and an ear to hear: he could show her things and tell her things, and taste the bliss of feeling that all he imparted left long reverberations and echoes he could wake at will”), Zeena sees her as someone who is not interested in the lessons she might have to teach, either about housekeeping or matrimony. Mattie is caught in the middle between these two unhappy people; what she needs to learn, of course, is the lesson neither Zeena or Ethan can teach – how to think for herself.