I don't think it's a "random antagonist to impede the relationship" considering 1: They're not in a relationship yet and B: Their living situation is nonstandard and hard to explain. This is the kinda drama I'd expect from the situation and fitting to the work.
It's also not interrupting progress/romance with a third character; he clearly communicated his feelings of being on her side and consoled her about her worries BEFORE the brother woke up and started trying to drag him off.
And its not got NTR/Cheating or a dark plot? It simply mentioned that the brother is distrustful of women because he was cheated on, explaining his over-the-top actions and reasoning; the sanctity of the main relationship's not been infringed upon in any way.
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?