I think it’s best for Shonen to leave this behind. Obviously it’s a manga, and is therefore a fantasy, but in my opinion in this scenario Shonen will fall into a pit of self-doubt, distance himself from his family and friends again, and focus on studying. He might burn himself out too. After passing the highschool entrance exam, Shonen will mature over the next few years and move on, opening himself back up to people and maybe even find a partner his own age. He might think back to his crush in middle school with rose-tinted lenses, and slack off sometimes. But overall, he will have little to no meaningful contact with Onee-San outside of the occasional awkward greeting and it will stay that way. Now obviously, this set of events will likely never happen and we will get a happy ending in the next 10 chapters, but that’s just my take on how it could go in real life.