beta
There's a high possibility it's not step bro, cuz the minhyuk guy was also wearing a white t-shirt underneath his blue sweater, and it might be author misleading us, cuz author-nim specifically pointed out him being weird with how involved he's getting with their project, he's acting "off" and the dress thrown around is not showing any black pants only beige/brown, which can point out to the Sunbae 's cardigan and the other's pants(which is brownish). And once Mc saw this stepbro will pop up and say along the lines of *tsk tsk, look what we have here, how scandalous, U mut be heart broken he's cheating on U" and then proceed to console him. Cuz step bro won't be able to get Mc if he's the one f-ing MC's bf, it wont make sense since they are kinda on a good bro term with each other, so if Mc got his heart broken and broke up with Sunbae, who do U think he'll be running to---> (づ￣ ³￣)づ step bro~ for sure and he'll console him with sex and manipulate him further into being dependent on him... And step bro will be like "see I told U, U only need me~" along those lines.
Just my 2 cents from seeing it, but I ain't still fully- reading it lolol