While it seems pretty clear to me, I'm not convinced that Catherine has any idea. Their whole relationship is pretty darned unbalanced if you ask me. Case in point: Chapter 6 begins by introducing a conversation that is said to be a "specimen of their very warm attachment, and of the delicacy, discretion, originality of thought, and literary taste which marked the reasonableness of that attachment." The next few pages then reveal an Isabella who dominates the conversation and speaks out of both sides of her mouth (the chasing young men situation that you mentioned, for instance), and a Catherine who just sucks it all in.