Ummm I don't think their actually brothers haha. Since this is a Chinese comic I'm assuming that "brother" in this sense just means good friend since alot of guys here call each other brother and girls call each other sister (even girls call their boyfriends "Gege" which means older brother) so this is probably just a way for them to say "best friends = brothers" if that makes sense! (づ ̄ ³ ̄)づ