@Chokai On a wild-wild-guess (that is, not reading a single page at all) I'll assume it's because this was here back in 2008 before the 'casual' meanings of "onii" or "nii"-chan reached the general audience.
THIS IS NOT INCEST. It's true that the girl adresses the boy as "big brother" but it's only because they have a close relationship and he is older than her. It's just a form used to expres respect/affection on her part.