Isabella ingratiates herself (very quickly) with Catherine by emphasizing the friendship of their brothers--recall that Catherine has not yet met Isabella's brother--and Isabella's acquaintance with James.

Catherine has grown up among straightforward people who love her (it appears,) and she probably understands friendly (family?) teasing pretty well; but the idea that good people cynically manipulate each other might well be outside Catherine's ken (based on what we see of her). Even if it were not, Catherine does not know about Isabella's hopes for James (so far as we see them in the first seven chapters), which makes Isabella's ulterior motives for "friendship" seem disinterested.