On one hand this manga isn't even in my top5 in that list.
On the other, I would probably never vote if not for this chapter, so it would be just to give it one of the votes.
Now I have to decide and that makes me sad.
@meidunk
It was mentioned at least three times up to this point though and kind of important for story and character development.
So yes, he is adopted and not to avoid incest.