Maybe it's because there's more of them, but I genuinely find Ai-chan's interactions with her classmates more interesting than with Onii-san at this point. He was already the least interesting out of the three original dudes and spent the least amount of time with his designated partner. It really doesn't help that their Monday morning train rides are the only connective tissue they've got, unlike everyone else who see each other on a truly regular basis. This lack of shared setting also makes Ai and Onii's age gap more pronounced imo. Maegami and Sensei never had that problem 'cause they were around each other enough for something to form regardless of their similarly taboo age gap. Ai, her friends, and the other students, on the other hand, have actually been a pretty funny and entertaining ensemble, all things considered. I'm sure we'll see her and Onii run into each other again eventually, but that pairing won't really take off unless their worlds have more reason to collide, so I'll enjoy the silly interactions of students being students until then.
All this to say... Kouhai-chan's group is BY FAR the best this series has to offer thus far. Kouhai-chan supremacy.