When a K-Firstborn Daughter Possesses a Childcare Novel

I Possessed a Childcare Novel.
In a rather ridiculous role.

“The eldest… daughter? Who takes after the cold-hearted Northern Duke?”

In the original story, I was supposed to be the eldest son and heir of the duke’s family—the one who adopts the female lead.
A stoic character who, despite his cold demeanor, is supposed to be warm and kind only to the heroine.
But that became me!

“Bellady, since you’re the older sister, you can give in, right?”

To make matters worse, my parents raised me with blatant favoritism toward my younger brother.

“As the heir, I command you—stop meddling!”

The younger brother, who was supposed to be the second male lead in the novel, treated me like I was nothing.

…Fine. Who cares if I’m an oppa or an unni?
First, let’s give this rude little brother a taste of his own medicine.

After restoring the hierarchy in the household, I decided to reclaim everything that was meant for the original eldest son:
The loyal retainers, the spirit’s treasure, even the title of heir.

And finally, my father brought home the young original female lead.

“This is Neshia, who will be the youngest of our family from now on. Come on, call me Dad.”
“I’m your big brother! Say it!”

At just 7 years old, the female lead was sweet and angelic.

Not that I particularly cared.
There were already plenty of other overindulgent family members to dote on her.
But… something’s off?

“Ah…”
“Yes, Daddy!”
“No, Big Brother!”
“…Unni♡”

The heroine ran straight into my arms, ignoring my frozen father and younger brother.
I haven’t even done anything for her…
Why does she like me?