No, no, no... this is "another" sister, not the missing one. Ophelia, Sophia's grandmother knows who the "Anonymous" donor of sperm for the impregnation of Annabelle is. It seems it was MC's father. With "seems" being the important part.
It may very well be a Red Herring for MB to play with our kinks, he apparently been studying Nyx's tricks (or teaching her, we don't really know). So, let's wait impatiently for the never issued confirmation.
Sophie, has too much loli to be attractive to me, but, if she's a confirmed half-sister, well, that's a whole new can of worms.
Peace