I find Henry's conversation with Catherine on Isabella and Captain Tilney's flirtation very interesting.

Firstly, society in general seems to place the blame on infidelity on the person who is not part of the marriage. These days women are seen as 'homewreckers' if they have an affair with a married man. In JA's day, the wronged husband could sue the man his wife was having an affair with. Catherine seems to disagree with this, as she thinks Captain Tilney's leaving Bath would solve the problem. Henry on the other hand is actually saying that it is not Captain Tilney's efforts which are so bad (although they're not exemplary), but Isabella's response and encouragement. In effect, he is saying that Isabella and women in general are not just unwitting pawns in men's desires, but are able to encourage and reject as they see fit. The implication of this is that women are not silly creatures, but capable of rational thought and decision. Catherine can learn from this that she doesn't have to like John Thorpe simply because he likes her.

Secondly, Henry's method of discussing this is interesting. He doesn't merely say 'Isabella is just as much responsible as Captain Tilney', but instead asks Catherine to question her own beliefs, and come to her own conclusions about Isabella's flirting and Captain Tilney's intentions:

And are you sure it is my brother’s doing?
Is it my brother’s attentions to Miss Thorpe, or Miss Thorpe’s admission of them, that gives the pain?
Well! Nay, if it is to be guesswork, let us all guess for ourselves. To be guided by second–hand conjecture is pitiful. The premises are before you

This has the effect of asking Catherine to consult her own reasoning, rather than asking Henry to come to her decisions for her. Once again, Henry is treating Catherine as a rational being rather than a silly creature.