Catherine listened with astonishment; she knew not how to reconcile two such very different accounts of the same thing; for she had not been brought up to understand the propensities of a rattle, nor to know to how many idle assertions and impudent falsehoods the excess of vanity will lead. Her own family were plain, matter–of–fact people who seldom aimed at wit of any kind; her father, at the utmost, being contented with a pun, and her mother with a proverb; they were not in the habit therefore of telling lies to increase their importance, or of asserting at one moment what they would contradict the next. (Ch. 8)

I don’t think James is ignoring his principles but has been taken in by JT as Catherine was by Isabella. James realizes JT is a rattle (to speak easily and noisily; empty and loud talk—Johnson’s dictionary 1824) as he tells Catherine in Ch. 7: “He is as good–natured a fellow as ever lived; a little of a rattle; but that will recommend him to your sex” I think his understanding of JT is just as limited as his sisters’ was of Isabella at first. The description of a rattle above is as good a description of Isabella as it is of John Thorpe. I think the main difference between them is their presentation. James sees JT’s loudness and exaggerations but puts no stock in it believing he has a good heart. Perhaps James admired what he perceived as JT’s easy manners just as Catherine admired Isabella’s in Ch. 4. In Ch. 9 Catherine is sure James does not drink as much as JT and her confidence is rewarded showing James still adheres to some principles despite his friendship with JT.

Isabella, in the meanwhile, had applied her handkerchief to her eyes; and Morland, miserable at such a sight, could not help saying, “Nay, Catherine. I think you cannot stand out any longer now. The sacrifice is not much; and to oblige such a friend — I shall think you quite unkind, if you still refuse.” (Ch. 13)

I think James’ betrayal of Catherine is the result of his affection for Isabella rather than ignoring his principles. He cannot see how his affection for her has compromised his judgment and values and caused him to do wrong by his sister. James is blinded by love. I daresay he has never been so bewitched by any woman in his life as he is by Isabella and is completely under her power. When she brought-on the tears he was lost to reasonable argument. I agree James may not want to see JT for the vulgar person he is since he is Isabella’s brother. I think it is probable James feelings for her gentle his view of JT otherwise he might have begun to see through him already. Catherine is beginning to see Isabella’s true nature because Isabella has let her guard down. She is no longer courting Catherine’s favor with superfluous attention, flattery and intimacy and she showed her true colors (as did JT) when Catherine refused to go on the drive to Clifton: “Isabella appeared to her ungenerous and selfish, regardless of everything but her own gratification.” Since love is more difficult to quell than a friendship of a few weeks it might take further egregious behavior by Isabella and JT for James to see through them—at least I hope he will for his sake and Catherine’s.