In the beginning I was hoping they were just close due to past events, then I hoped the incest wouldn't progress any further, but now I don't care anymore. I actually find funny the scenes where they get all mushy with each other, and one of their friends cracks a joke about it.

Besides, Tatsuya is broken as a human. Maybe not as broken as the Generator assassin, but not too far away, and I got the feeling he was made that way on purpose, likely by the family itself. Although there's also the possibility he was dying and it was the only way to save him (if that's the case, then it was to save Miyuki, no doubt). It seems like he can only feel anything for Miyuki, so I find it kind of sympathetic she's trying to grant him all kinds of feelings. It would be better if she wasn't so willing and eager to commit incest, but would be doing it nonetheless for Tatsuya's sake, to keep him a bit more human. In fact that would have been a novel element in fiction for me, I think.