Are they actually brothers or does he just call him that cause they're close?

They are not brothers.

they’re not actually related at all, the korean language has the term “hyung” (older brother) and it’s used between male friends too, not just blood-related older brothers. the translation just translates it to “brother” instead of keeping ‘hyung’ like other manhwas lol.

Thanks for answering! Glad they're not but now I'm confused on how they both have a right to the throne of they're not related

they are related. just distant. richard is a family member of etienne's father, the emperor, and thus has a right to the throne. since etienne's mom killed all other closer relatives, being like 15th in line for the throne turned into 2nd lol