I hate to say it but I think James is not watching out for his sister as much as he could. He told Catherine that Thorpe was a rattle, someone who chatters without much thought, but he also curses often and is very conceited, prejudiced and full of very foolish notions. James seems to be blinded by Isabella’s attentions to how little Catherine enjoys his friend’s attentions. I think James has never seen a lady like Isabella and is entranced and a bit naive—that is the only way I can explain his letting Isabella and Thorpe persuade Catherine to go riding when she obviously does not wish to. :D