The Atmosphere of the Novel Is in Crisis

I was transmigrated in a novel that was written by my older sister. But... the atmosphere of the novel is a little strange?

After my older sister got in a car crash, I was granted an opportunity to fulfill one wish in honor of her good deeds.
So I wished to become the character my older sister cherished the most in a novel.
But I then ended up possessing an antagonist named 'Satin' instead!
But the complications didn't stop there... The main character 'Cain', a holy knight, keeps interfering with me.

Even though I was not really a big fan of novels. I still tried to dig into the uncovered mysterious secrets of the novel, and play the 'faithful villain.'

But... my relationship with Caine started to become deeper than anyone else's...
And now everything's spiraling out of control!!

(Source: MangaUpdates)