Country-born inexperience is an excellent way of describing the Morlands but as you say James has had plenty of time to compare the behavior of John Thorpe with that of others with appropriate manners and behavior, not that he should need that as Catherine sized Thorpe up right away.

If nothing up until this time has given him a clue that the Thorpes are bad news, their bully beat-down of his sister in chapter eight should have done it.

When Catherine refuses to give up her walk with Eleanor, Isabella urges her to break her promise and to lie while doing it. When Catherine refuses to do this, Isabella uses sweet talk but Catherine, "could not allow it to influence her." The next tactic is to begin a guilt trip by making accusations of not being treated as her best friend. Catherine thought this was unkind of Isabella and James steps in and accuses his sister of being the unkind one, '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.' This was the first time of her brother's openly siding against her..."

Catherine tried a compromise: she would go with them the day after her engagement. No. She made a suggestion: one of the Thorpe sisters could go. No.

Thorpe goes off but as the other three continue walking, Catherine, "was again attacked with supplications or reproaches..." Ten minutes of this until Thorpe returns with the news that he told Eleanor that Catherine had sent him to tell her that she had forgotten a former engagement with them and would go for a walk with her a day later---a lie. "Isabella's countenance was once more all smiles and good-humour, and James too looked happy again." I mean, really?

Catherine is upset and says she must run after Eleanor and set her right. Isabella catches hold of one of Catherine's hands and Thorpe the other to stop her from doing so. Not only is James allowing this but, "the remonstrances poured in from all three. Even James was quite angry..." Seriously? Even in this day and age, a person would feel imposed upon if two people constrained them like that. And during JA' s era a woman doing this was bad enough but a man was not suppose to put his hands on a woman except under certain circumstances and this was not one of them. What was James thinking? I am sure he was taught right from wrong the same as Catherine. If his parents could have been flies on the wall, no doubt they would have been disappointed. I personally find it more appalling each time I read it.

Poor Catherine must have come off as a crazy person at first dashing past the servant and bursting into the room the way she did and immediately diving headlong into an explanation that was difficult to piece together. However, it was a good thing that she managed to straighten it all out as there seems to have been some resentment over her making a second engagement and five minutes later canceling it (or so they thought).

One other thing that has always bothered me, the idea of going out in an open carriage for a third time in such a short span of time was not proper anyway. James should not have been willing to compromise his sister's reputation like that nor Isabella's for that matter even if she didn't care.

I really can't interpret this any other way than that James was willing to compromise the principles with which he had left home with while Catherine was not. I think it says a great deal about both of them.