Wut?

You said it yourself. she’s a bitch

Because he was sleeping with her boyfriend (at least he thought he was). So basically, in his mind, he was sleeping with his best friends boyfriend. The fact the boyfriend is married and a side piece just makes her a hypocrite, but I can understand the thought process. She's prob also pissed because she got dumped and he's the only one she can kick around right now since the bf prob blocked her

I'm struggling to see things from her perspective, as even if she accepted that Junyoung was upset because she thought Shihoon was her "married boyfriend oppa" and not her "brother oppa", surely she has an ounce of self awareness to understand what a hypocrite she is being, and it was she who deserves to be ashamed of herself given how she acted in front of her brother and her "married boyfriend oppa's" wife. As you said, she probably got dumped by her "married boyfriend" oppa, and Junyoung is the only one who she can take this out on. Her brother, from what it seems avoids conflict, and rarely visits his parents home (where presumably Mari is still living), or in earlier chapters, where as a younger man he did not want to "come out" as gay, meaning his boyfriend at the time left him, and moved to the USA where he met someone who did not want to hid e their relationship.

I also genuinely do not understand why someone would dislike my comment above. I was just asking..

They probably miss the like button, it's so easy to do that when you're typing on the phone

true true