To be fair, that coupling is probably the most problematic of them all, so focusing on the others has more general appeal.
Ai is like 16-17 while old man is like 30s - it's a bit uncomfortable.
Yeah, nowadays I can't really get into Ai's relationship with "nii-san".
I know this is fiction... but I'm a man who has reached his 30s, when I think about a man looking at a girl half his age like that, a teenager girl no less... it is really uncomfortable.
I don't preach anything, I don't want to convince anyone, like I said... fiction... but I won't be shy about my opinion on that either.