So not nearly as bad as setting things up, but still self-sabotaging in his refusal to talk about what’s happening or why he’s upset. Arrrgh…
Hell, even a, “Hey, your instructor is my brother, sorry I didn’t tell you before. He has a history of sabotaging my relationships and hobbies, and I think he’s targeting you to try to hurt me. Would you please not see him outside of class?” Like… say the thing. So much easier for all concerned than what I fear will happen next chapter.

Did Jaeyi know his brother was going to be the instructor for the class, or was he substituting for someone else? I can’t remember. (I think he took over last minute…?)
If he knew, did he set this up as kind of a subconscious “test” to see if Ideum would/could be into his brother?
Either way, it seems like a terrible idea to have set him up to swim at the place his brother works if he had any idea he would be there. Almost like self-sabotaging with his fuck buddy as “bait” in a longstanding rivalry. It’d be incredibly screwed up of both Jaeyi and his brother to involve Ideum at that point. I think I must be misremembering.
But anyway… I am once again begging characters to talk to each other. This could be cleared up in like two sentences.