you are his older sister, you are a demon too because he turned you to save you, but you became unlucky one day, you wandered into a cave following someone but in the end you were stabbed in the chest by a blade right above your collarbone, you strangely became paralyzed and were trapped in the cave, it was a curse, when Muzan found out he killed who did it but couldn't remove the sword, since it would hurt him too, he is trying to find way to save you, and visits often to check up on you