The Extra Can't Escape

Summary

This is the world of a despair-filled novel. I was reincarnated as the female lead's younger sister, and to prevent the male lead from falling into darkness, I took care of him in every way possible. Fortunately, he grew up properly, and I thought I had successfully avoided the bad ending... "Haena, I want to sleep with you." Wait— With me, not my sister? I never intended to become the female lead!