"Omae ga Kawari ni Shine" to Iwareta Watashi. Imouto no Migawari ni Reikoku na Henkyou Haku no Moto e Totsugi, Shiawase o Teniireru

Both my parents had always only loved my sister and I had no place at home. I lived my life believing that as long as I worked hard, I would someday be loved. One day, my sister received a royal order to marry a margrave who was rumored to be ruthless. My father said to me, "You will marry into the margrave's territory instead. But on the way, the carriage will be attacked by demons— you will die." As planned, I was attacked by monsters, but the one who rescued me was the ruthless margrave, a beautiful man with bright red hair and sharp golden eyes...

Note: It began as a web novel on Syosetu starting on March 29, 2022, before later being published in print by Futabasha on February 10, 2023.