The Extra Can't Escape

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!