Well, the plot could involve her being a secret power player in the political intrigue of the Code Geass world. Maybe she has her own Geass ability that she keeps hidden. She could be working behind the scenes to either support Schneizel's plans or have her own motives for the future of Britannia.
Another idea is to create a story where Schneizel's sister gets involved in the political intrigue. She could be a powerful force, using her intelligence and charm to either support or oppose Schneizel's plans. For example, she might try to find a more peaceful solution to the conflict in the world of Code Geass than Schneizel's more extreme methods.
First, consider her personality traits. Is she as calculating as Schneizel or more emotional? You can give her a distinct personality that sets her apart. For example, if she's more empathetic, it could lead to interesting conflicts with Schneizel's cold - blooded strategies.
Her character could be developed as a highly intelligent and strategic thinker, just like Schneizel. She might start off as a somewhat mysterious figure, with her true intentions hidden. As the story progresses, we could see her making difficult decisions that show her moral compass, whether it aligns with Britannia's values or goes against them.
Yes, it is. As a 'Code Geass' fan, it gives you more content related to the series. You get to explore the story further and might discover new aspects of the characters and the world.
Code Geass did start as a manga. However, its anime version became even more widely known and beloved. The manga provided the foundation for the rich story and characters that were further developed in the anime.