My Sister Dotes on Me, While Her Younger Boyfriend Has Been Plotting for Ages

Transmigrate into a book + System + yandere + redemption + intense push and pull + happy ending

[Obedient puppy at first, then a dark, twisted yandere bad dog VS a wicked on the outside, clear-headed young lady within]

Sheng Zhuxiao transmigrates into a book as the wicked supporting female character who torments the yandere male supporting character.

She has been here for five years, but his corruption score hasn't changed at all. The System says this world is in danger of collapsing.

So...

"Make him your servant, and have him do your laundry every day."

"Sister's clothes smell so good. I love them so much."

"Humiliate him. You're just a dog I keep."

"I'll gladly be your dog for the rest of my life, Sister. Just don't abandon me."

"Mistreat him. Make him eat the food he hates every day."

"I'll gladly eat your leftovers, Sister. If you don't mind, I can even eat straight from your mouth."

The System thinks that's still not enough. She has to humiliate him to her heart's content and fill him with hatred so he can grow into a proper big villain.

Sheng Zhuxiao: ???

,

Sheng Zhuxiao finally completes her mission with painstaking diligence, thinking she'll return to modern times and live like a rich young heiress.

But after faking her death, the System pulls her back just a few minutes later.

"Five years, Sister. You're finally back..."

"Sister, I haven't forgotten a thing you taught me!"

"If you like her, take her for yourself and lock her up."

The once-lonely, helpless puppy has grown into a yandere, deranged, obsessive wolfhound.

"Be good, Sister. Seeing you cry only makes my heart race more."

P.S. Younger man in the first half, older man in the second.