Chapter two of Edith Wharton's Ethan Frome, we discover Ethan's feelings for Mattie Silver are certainly more than platonic, though it is equally clear that he has done virtually nothing about those feelings. He is thrilled to have time alone with Mattie as he walks her home from the dance, and Mattie seems to be enjoying their time together, as well. When they arrive home, however, something is not quite right.
Mattie is Zeena's cousin, and she has been living with the Fromes both to help Zeena but also because she has nowhere else to go. When Ethan and Mattie arrive home tonight, they do what they always do--the look for the key which Zeena always leaves for them under the mat. Zeena goes to bed early, so this is the routine they have established. Ethan bends over to collect the key.
"It's not there!" he said, straightening himself with a start.
They strained their eyes at each other through the icy darkness. Such a thing had never happened before.
"Maybe she's forgotten it," Mattie said in a tremulous whisper; but both of them knew that it was not like Zeena to forget.
As he continues looking for the key, Ethan has a fleeting thought that perhaps someone has broken into his house and done something awful to his wife; however, he does not allow that rather exhilarating thought to get very far. Finally Zeena opens the door. She did not forget to put the key under the mat; she said she "just felt so mean [she] couldn't sleep."
The next thing she does in this chapter is also out of character for her. Ethan says he is going to work on the accounts before coming to bed; in fact, he just does not want Mattie to see him follow his rather awful wife up the stairs. Instead of going on to bed herself, Zeena waits for Ethan and more or less shames him into going upstairs with her instead of letting him stay downstairs with Mattie.
It seems clear because of these two rather defiant acts that Zeena suspects something about the relationship between her husband and her cousin.