How close they are is finally revealed in ch 15;

The announcement of James' engagement to Isabella,

Catherine wished to congratulate him, but knew not what to say, and her eloquence was only in her eyes. From them, however, the eight parts of speech shone out most expressively, and James could combine them with ease.

That Catherine loves her brother we know, but whether he loved her equally was in doubt.

Such communication between the two is proof of their love for each other, and finally I am ready to fully forgive James for being so preoccupied - which I think is so natural a thing to do.