I Am Sorry, but the Ending Is Already Messed Up

I woke up after everything had already fallen apart.
The game’s main protagonist was gone, and the protagonist’s fiancé was consumed by anger, avoiding everyone. In the country, a deadly disease was spreading.
The game’s ending was the worst possible, and I found myself in that awful world as the twin sister of the main character—an unfortunate ‘villain’ who caused my sister’s death.
“I’m in big trouble.”
My new life was ruined before it even started.
I was Fey O’Bellie, the twin sister of the main character, born into a prestigious family. But titles held no significance for me. I was merely branded as the ‘villain’ responsible for ending the life of the main character.
The ending was final and unchangeable. All I wanted now was to stay alive and keep quiet.
“Did I actually end Luné’s life solely because she was foolish, arrogant, and insufferable?”
Survival seemed impossible. Someone knew the truth and wanted to kill me—the stepbrother who loved the main character Luné, Deneb.
I have to get away from him.
…In a world where I was the cause of Luné’s death, someone loved by Deneb, how could I survive?
“Even a character from a messed-up ending like me wants to live, too, you know.”
Alright. I’ll escape then.