Yeah...but what set me off was that other girls knowing stare.
Shes perceptive- i think that's the reason she was created in the first place- and she was able to tell that something was amiss when he was talking about tsubaki.
I don't think he's a creep or anything like that- just...maybe a bit too... obsessed?
Mmm I read that scene as her realising the child wasn't comfortable with her and she was very kind for leaving instead of trying to force him to like her, like many older people do with children. That being said, I can't deny that on a second thought I felt it sounded like that random character who understands the real feelings of mc, but realistically I'd say it's more likely she thought he was shy or "jealous" of the bro's attention (like children do).
I think it's ok if he's overprotective, as long as his feelings are just brotherly.

I really hope the older dude doesn't have feelings(romantic at least) against tsubaki.
I do like the trope of the younger childhood friend carrying a torch for the older, oblivious tone, but he has to be exactly that. Oblivious.
Otherwise it's...pretty scary, not gonna lie