in the Chinese language, and this applies to many other asian languages, terms such as brother, sister, aunty, uncle, are used for pretty much everyone but it's clear it isn't meant in the familial way. however in English "brother" would be the direct translation, and we don't use these terms to address anyone else besides family so i get how it seems a lil wack.
it's similar to "hyung" in korean, u use that word towards an older male, especially when you are close to them. Doesn't necessarily mean you are related.
i think he is calling him "ge ge"
Which means older brother..
If you watch chinese drama, it wont be weird for you.. because you dont have to be brothers to call someone "ge ge" (older brother) just like what salty sempai said ..
"Ge ge" is like hyung or oppa in korea
It’s so weird when they call each other brothers