This whole talk about "we learn to love because of you, mix-girl", is giving me some serious incest warning bells.
Realistically, this is actually a pretty great trope to use to do just that, and focus an entire story on a family that needs to lay with their own blood in order to be happy. Hibby jibbies.
@collagen
This is is gonna be a necropost, but I got an answer for ya.
Try to think back to the earlier chapters, in which the brother was ill. He grasped at the girl, and told her that he instructed everyone in the mansion not to visit him. I believe that shows that regardless of what they say, they are still uncomfortable with humans around, but not to the extent that they cannot get used to it.
The dad seems much less bothered by them. He did have sex with a few human women, after all...So it is something that is expected of the brother to grow more accustomed to as well.