It seems to me that Isabella doesn't know her brother as well as Eleanor does her own, because it seems that Isabella (and her mother, etc.) have often taken what John Thorpe said seriously, rather than seeing for what he is. In fact, Isabella may believe John as suits her own desires, and it created a snowball effect. Here's a hypothesis (can't confirm or deny it, but it can be debated): James Morland was one of the few blokes at Oxford who could tolerate John Thorpe, just because - like the rest of his family - he is disposed to think well of everybody at least initially. John finds it remarkable to have someone who really listens to him and believes what he says, so he flatters James and thinks he's a right good guy. He invites James home with him for the holidays and introduces him to the whole family as a really good guy - a good guy to impress the family and to puff himself up. So he indicates to the family James is a nice guy and has money too! Isabella latches on to that idea, because she wants to believe it. John is happy to keep reinforcing it, and it snowballs. By the time they get to Bath, because Isabella is reinforcing an attachment to James, and John has no reason to try make his only friend look bad, he won't investigate it, and will perpetuate the lie. Talk about traits that shoot an ego high into the sky, only to cause a total stall and tailspin in the end. I tend to agree with BarbaraB on this one, although I can understand why it's hard to believe it all started from nothing!