Another thing to consider is whether the sister is a character from the original series. Mao's sister isn't a major character there, but maybe in the story's context, she could have a sister. Let me check. In the original, Mao's mother had another child? No, Mao is the daughter of a prominent family in Kamui's universe, but her sister isn't mentioned. So maybe the user is creating an alternate universe where Mao has a sister. That would make the story original. Alternatively, they could be thinking of another anime or manga where a character named Mao Hamasaki has a sister, but I'm not aware of such. Therefore, it's safe to proceed with an original story based on the Kamui universe but adding a sister character.