The Bros. Knightley are indeed keen observers of human nature. Re. Mr John Knightley, his good-natured teasing of his hypochondriac father (who obviously brings out that aspect of his wife's nature) is actually endearing as a coping strategy, especially since we're told that after such episodes he goes out of his way to be extra-especially nice to everyone. It's easy to see why Isabella "doats" on him.

As to the psychological penetration of Mr George Knightley (THE Mr Knightley!), it seems that he's onto one of Emma's secrets, rather than (as his brother tries to do) revealing a secret to her. In Chapt. 14, we've already seen (via the ON's peering into Emma's brain) that Emma has a little marriage fantasy about Frank Churchill. This makes sense, of course, given her fondness for the Westons, and the fanciful, "what-if-this-happened?" nature. In the long dialogue scene that ends Vol. I (chapt. 18), Emma protests that Mr K "seems determined to think ill" of Frank Churchill, and is privately surprised "that he should be so angry" on the subject. Although Mr K protests that this is largely due to what he considers a snub to Mrs Weston, it doesn't seem far-fetched that he considers Emma's over-interest and over-readiness to make excuses for FC as a bad sign.