Is he Jaeyi's brother who he has a complex with? Vice versa??

Yes!

Explain

In chapter 10, Jaeyi’s classmates talk about his brother, says that he’s a retired swimmer, and they pull up Kwon Jiseong’s athlete profile. The face is hidden but it is the same spelling. And then the swimming instructor says, “you know him to?” He then talks about Jaeyi, which implies that Jiseong already knows Jaeyi lol

P sure the “you know him?” is about Ideum. Someone who knows both Jaeyi and Jiseong would know they're brothers