You might wanna reread what was written.
1. Introduce random antagonists to impede the relationship
2. Interrupt any kind of progress or romance with a random third character walking into the scene.
3. Introduce random NTR, cheating, or dark plot into your Comedy Romance.
1. Indeed the brother is impeding the relationship of the MC/FMC infact in this very chapter he impeded them having a relationship building moment.
2. 1 and 2 are kinda the same point tbh but for no reason we have the lil bro sleeping in the room with the 2 of them like why would this be?
3. Random NTR did happen to the brother as a plot point. Nothing said it had to be the main couple for this point.
My thoughts on this chapter are what terrible crap is this? MC just lets his brother do all this crap? Why is he letting him pull him around? Why would he immediately go against what he said to FMC? This series while a little jank with a dense MC was ok and now it's sent it all down the gurgler. What next he'll have a drama arc of moving out with the little bro cause MC is a wet blanket that folds faster then origami?
Been off MD for a bit, but my response:
1: You ignored the first point and conflated it with the second; the original first point is about the brother only existing to 'impede the relationship', as the new chapter is pointing to his weakness has served as a strength for it. MC learned something potentially negative about Female Lead and still supports her unbiasedly when seeing her in a situation easy to misinterpret for what the brother feared; that shows a deep level of trust and respect.
2: Again, the progress was made by that point, having the brother drag him off only served as a way to establish the next step of the story, not impede it.
3: When people complain about NTR in a romance work, its about the main lead. Her having had an ex as we now know, or a potential male in her life is likely the NTR the original commenter feared; they would've believed the brother was negative foreshadowing for her being in a relationship with someone else, but that was just an assumption on their part, hence the line about the main relationship not being infringed upon.
4: You're also kinda guilty of making wild assumptions based on a development of a single chapter here bud. Yeah, its unfair for me to say that now that my points have been confirmed but I woulda said the same even if your assumptions had been proven right by now; while you can try to infer what happens next its not fair to authors to assume that something you dislike in this chapter means the characters have been flanderized permanently.