My Sister and the Cat and My Life

[Light novel × first person × depression × fantasy × single female protagonist × brother and sister] A black cat led me to my sister I had never seen before. I bought her snacks, taught her math, and listened to her sad stories. She is a relative I have forgotten for many years and the only person I want to protect in this life. It wasn't until the curse of fate appeared and the buried history books were picked up that we understood—— Our encounter was a fateful punishment. I am willing to give her half of my life in exchange for her seeing the world one more time. Even if it is as a brother.