Because it's not incest, it's step-cest in the true porn kink sense. The game started out using real porn assets and actresses (you might even find the first version on this site). Although I don't remember the actual dialogue the first time Minami was introduced, her story has always included the reveal of why she was adopted (when her story was completed).
So, yeah, it's not actual incest, it's actual step-cest which might have some common themes to incest (living under the same roof, same or similar parents) but so do all the childhood friend neighbor themes.