Kujibiki Tokushou: Musou Hāremu ken - Chapter 43

Once an imperial princess, now reborn among the lowest of s*aves of the nobility.

After choking to death on a watermelon, Rong Jiahui thought that she would enter the netherworld and be reincarnated.

But when she opened her eyes, she discovered, to her very horror, that she had returned twelve years into the past…

Looking at the beautiful and awkward “little brother” that would be conferred the title of Marshal in the future for outstanding military service, she almost let out a snort of laughter.

She firmly decided to hug the thigh of this little brother, and climb to the top of her life!

The plan was beautiful, but it had just one problem.

Why had this little brother… suddenly turned into…

A little sister…

- Description from Novelupdates

Jiang Xiaoman was bound to a system when she was three years old. The system told her that she was the lost daughter of a wealthy family assigned to play the cannon fodder in this script. The fake daughter that her parents adopted was the heroine of the story. 

Jiang Xiaoman blinked her eyes and asked, “What is a cannon fodder?” 　 

System: “You all live in a book, and everyone is assigned their role. From now on, you should keep on blackening and be a bad child. Do you see that little boy? He has a bigger role than you, in the future, he will become the big bad villain. Why don’t you bully him now, and show him up real good!” 

The naïve Jiang Xiaoman picked a handful of colorful flowers and stuffed them into his hands. “I’m showing you something really good!” 

System: “…”

Darling, you got it all wrong! Not showing him something good that way!

The little boy: “I don’t need friends, I don’t need company. I’m just a burden that no one wants. It’s my destiny to be abandoned.” 

Jiang Xiaoman: “That’s wrong. Everyone’s existences have meaning. System said you are especially important, even more important than me. You’ll be a really really useful person in the future!” 

System: “…??”

Did it say that?? 

Later on, the system stopped giving Jiang Xiaoman any tasks. Jiang Xiaoman was very sorry and said, “Mr. System, why won’t you play with me anymore?” 

System: “… just try to grow up first and learn to understand every word! You’re killing me!!” 

Jiang Xiaoman worked hard to blacken (not really) under the system’s supervision until she was reunited with her wealthy parents. She scratched her little head and asked, “Mr. System, what about the task? Didn’t you say that we should fight over the family’s wealth?” 

System: “What kind of wealth do you still want to fight over? You already have more money than your wealthy parents!”