Even though its his brother remember that he left him as a 8 week old kitten, in human terms that about 2 years old........if you then met your brothers/sisters a year later (human terms I think is 18??)
Would you remember them???
There will be lots of different smells and cats dont like change, keep him in one room until the first week or 2 until he calms down and no meeting his brother, hell know his there but its too much for him right now, he will be able to smell him, but do supervised intros for 15mins a time & give both there fav treat so they think of eachother with nice things!
OH just re-read that they split up at a year old, it could be the smell the distress of everything.
Cats dont forget though, my cat KNEW that he had injections every month same date...he would go missing and not come back until it was dark - he knew that we wouldnt take him then itll be too late.
Not stupid lol!