This letter appears to be a drawn out detailed set of insults to his older brother. Maybe cultured isn't the best translation. Is this what we in the US would have called "classy" back then?
http://allowe.com/laughs/book/When%20Insults%20Had%20Class.h... - just a quick hit on "classy insults" that seems to fit what I'm saying here.
> 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.
Sounds like his brother probably satisfies at least 1, 2, and 3. That's why I say gentle; he says "you're a great guy, you have natural talent and the potential to be cultured, just need to work on your attitude and develop your talent".
In this case, though, I believe he was actually frank, if blunt, but well meaning (his brother had issues with alcohol).