before i say anything, let me say that irl grooming is not ok. it's creepy af and people who do it are pervs.
that out of the way, this isn't real life, and real life logic hasn't applied so far. Every character that's aware of their relationship either think it's ok, dismisses it as "kinda weird, but ok" or downright encourages it. I'd be very surprised if the author introduced the reprehensive moral consecuenses of dating a minor into the balance at this point in the story.
I think we're due a more detailed view into onee-san's pov. We've been given glimpses at best so far, we need a better perspective on how she really feels at this point, because shounen's feelings are on his sleeve now.