Board Thread:General Discussion/@comment-213.140.59.150-20140925181645/@comment-1348186-20140926061940

A child's succession position is determined not by their gender, but by their mother's status in the Imperial Court. Thats why, for example, Marrybell(from the Oz the Reflection side story) is 88th in line, despite being older than Nunnally and Carine. But yeah, its not just the only thing that matters as shown by known Britannian history.

As for Cornellia, she has stated herself that she has no interest in taking the throne, so even if she is qualified by birth, she doesn't want the position. Nunnally gets the position as Empress because she is the only one both left with the right and willing to accept it, as all other known living royalty are either Geass's or not interested.