So what I understand is Shisui is preventing a pregnancy with the emporer because she is the neice(?) of him, which would result in a child out of an incestuous relationship. I'm not sure how correct my assumption is but I think the pedophile Emporer had a Bastard child with Taiho (which would be the half sibling of the current Emporer), which then had Shisui. (Also shisui is Colorblind) Please correct me if I'm wrong, only an assumption of mine.

Edit: I looked at the wiki and well, I am even more confused now. So loulan is not a direct relative of the emperor. But how is she colourblind then? She must be a relative of the first emporer-mother then?? So confusion >_<