I can't tell if Siwoon actually said "you're so fucking annoying" or did Jiwook imagine him saying that, thats quite important I think as to whether Siwon was a good brother and Jiwook is just jealous, or is Siwoon a conceited bully
I think the way the artist drew it, it's in Jiwook's mind (same as what the adults said in faded dialogue). Its' what he assumes they are thinking/saying about him.
I can't tell if Siwoon actually said "you're so fucking annoying" or did Jiwook imagine him saying that, thats quite important I think as to whether Siwon was a good brother and Jiwook is just jealous, or is Siwoon a conceited bully