I still dont know if they are brothers or not lmao ╥﹏╥

ikr me to

I don't think they're brothers because the author said in her tweets that the story is about their lives: their love, their hatred, so on and so on.
She also has referred to them as couples a few times as well.

Her tweets:
https://twitter.com/vpej12/status/1154744901067628544
https://twitter.com/vpej12/status/1154747393838964736
https://twitter.com/vpej12/status/1175114229684170752
https://twitter.com/vpej12/status/1219433755770683392
https://twitter.com/vpej12/status/1231786795290902528

A few are in Korean, but when translated she calls them a couple. I hope this is helpful.

thank you <3

Sometimes the translation shows them calling the other 'brother' but I think its just him saying 'Hyung'.