In addition, the WN reveals the emperor is only an adopted son of the previous emperor.
He has less bloodline legitimacy to become monarch compared to Cedric's mother and Roygar, who were the previous ruler's biological children.
Emperor only married the empress for her family's ducal title to improve his own legitimacy to get the throne, but killed them later on to prevent them interfering with his power base. Because his legitimacy is tied to her family name, he can't just get rid of her and make Miraila empress (since Miraila is low in the noble rankings). What he can do is cut off the empress and make her powerless, so he can freely pursue his relationship with Miraila.