Responses
actually i looked at the raws and he says "아우야" which roughly means younger brother in the dictionary
They may have just been raised as brothers or he calls him that as endearment because later in theie conversation he clarifies He is Bihwan's father's nephew, making them cousins.
Guys they clearly say they are cousins Bihwan says that Muyeol/Gwonryeom still thinks of his (Bihwan's)father as an uncle
the brother thing was probably literally translated from "hyung" (which means bihwan may be older than muyeol if my guess is true)