@lovehydration If it's anything like the 14-sai no Koi side story, they'll just start dating like it's the most normal the most normal thing in the world and even move in together eventually and everything will be hunky dory, right up until the MC pops the question and it's only then the FMC seriously starts to think about their age difference, but only from the point of view of how large the gap is between them, not that they've been fucking since he was in middle school. It's fiiiine.
In this case, while I suspect the author will probably hand-wave it here too when things start heating up, especially given that Shounen's mom is already trying to set them up, I have a feeling it'll be treated fairly superficially and innocent. Even with Onee-san's personality, I wouldn't be the least bit surprised if it turned out she has no relationship experience and it becomes an age gap Kawaii Hito but with an abundance of slacking off. Of course, the author could also just not have it progress, but we already know Shounen has a crush on onee-san and he's starting to realize it, so it's only a matter of time before something gives.