There is lots of manipulation going on in Ethan Frome. The more desperate the characters become, the more they try to manipulate each other. Often, it's not clear exactly who is manipulating whom, and perhaps even more importantly, what they hope to gain. This is a particularly ugly aspect of the novel, and works with the theme repression. Because the main characters are so repressed in almost every area of their lives, they often fall back on manipulation as a substitute for more direct forms of communication. Zeena, Ethan's wife, is the main embodiment of manipulation, and she uses her health. Her health changes and can be good or bad seemingly depending on her needs at a given moment. But, as Ethan and Mattie Silver fall in love and try to hide it, they also fall into acts of manipulation of both themselves and others.
Because Ethan feels he can't openly ask Mattie how she feels about him, he tries to manipulate her into telling him.
Zeena uses manipulation because she feels powerless to express herself in other ways.