I might say "gentle criticism" rather than insults, but the beginning and end of the letter do make it clear that he is specifically advising his brother: http://www.gutenberg.org/files/6408/6408-h/6408-h.htm#link2H...

> You have only one failing, and the falseness of your position, and your unhappiness and your catarrh of the bowels are all due to it. That is your utter lack of culture.

>You must drop your vanity, you are not a child ... you will soon be thirty. It is time! I expect you.... We all expect you.