It isn't entirely clear which part of the novel you are referring to. There are quite a few sections where Catherine plays off her husband Edgar against Heathcliff, which reflects her selfishness and also the relationship between Catherine and Heathcliff which seems to be more based on mutual self-harm than mutual love and building each other up. However, in Chatper XI, Catherine goes into a frenzy because of Heathcliff's attentions towards Isabella, and Edgar's threats of stopping him from visiting Catherine. It shows how Catherine uses her behaviour to manipulate others and get her own way. For example, Catherine locks herself in her room saying:

"And, Nelly, say to Edgar, if you see him again tonight, that I'm in danger of being seriously ill. I wish it may prove true. He has startled and distressed me shockingly! I want to frighten him."

Here we see the self-centredness of Catherine - she does not feel she is in any way to blame but wants to be able to control others. See what she decides to do if she cannot keep her acquaintance with Heathcliff:

"Well, if I cannot keep Heathcliff for my friend - if Edgar will be mean and jealous, I'll try to break their hearts by breaking my own. That will be a prompt way of finishing all, when I am pushed to extremity!"

Catherine appears almost childish and petty, and certainly over-dramatic with this speech and plan. Note how Nelly, being aware of her plans, is not impressed by her dramatics and convinces Edgar that her "illness" is nothing to be concerned with either.