
There are here 2 Oppas
"OPPA" - guy who have sex with main character, he is not married, he don't have girlfriend, he is the brother of the main character's friend (long hair girl)
and
"OPPA<3" - the guy (we don't know him in that manga yet) who is with main character's friend (lol, sugar daddy), and he is married with woman (she was in the first chapers)
it's one big misunderstanding because main character thinks "OPPA" is "OPPA<3" and he thinks "OPPA" has wife and he is with main character friend

Umm, did you maybe confuse the terms for uke and seme or did you nod read properly? Just in case: Seme is the active / top part of a couple, while Uke is the passive / bottom part. The uke in this manga has no sister, as far as we know. His seme has a sister, which is uke's best friend. But uke thinks his Seme is his best friend's lover.
so is he married or nah? who was the woman that slapped junyoung in the beginning? is she not the wife