You would think that Seiha, being the little bookworm dweeb that she is, would've at some point ran across the hypothesis called the "Westermarck Effect" (a.k.a. reverse sexual imprinting) somewhere in all of her research. It's the psychological hypothesis that states that people tend not to be attracted to peers with whom they lived like siblings before the age of six. Plainly speaking: In this sort of setting, having been raised together since childhood, the idea of ANY sexual attraction between Arata and any of the girls--blood relative or not--should ipso facto be VERY off-putting to any of the parties involved.