Obanai's Sister?!(Fem!Feitan!Reader/Oc)

Kimi Obanai is the sister to Iguro Obanai. She did become a demon slayer but during one of her missions she met the demon king. When meeting the demon king he knew how much potential she has to become a strong demon so he asks her to become a demon. She thought for a moment then she decided to make a deal with the demon king. What kind of deal you may ask? Well the deal was if she joined him she wont become a demon but will help out the demons without her brother and the other hashiras knowing. Her brother hardly cared for her so why would he care if she joined the demons side. Iguro never told anyone even Mitsuri that he had a sister. They never knew she existed. When going to the Obanai estate he would hardly care for her and would sternly ask for her to give up her dreams. She would just ignore him and leave the estate and go demon hunting since she didn't want to deal with her annoying brother.