Muzan x sister reader

Brother and Sister

Ever since Y/n was born, the life and atmosphere of the Kibutsuji's estate became dull and dark.

The staff and even the head of the family believed that they were cursed due to the fact that both of their children was weak and were always at death's door.

At first you thought it'll be ok, since you have Muzan's back and he got yours. But since he was a male heir... He had more importance than you, a daughter.

Sure, both of you had same medical treatments and lifestyle. The only thing you don't have was your parents attention.

Despite that, you never held anger nor resentment to your poor sick older brother.

"Muzan-nii?" Your soft small voice called out. Since you were so soft spoken, every single one in your estate was annoyed, and this doesn't exclude Muzan.

He didn't turn to you. You tried to call out louder but to no avail, your internal organs are so sensitive that too much work can be troublesome.

You sigh as loneliness sinks in. Even if Muzan was sick, he never stopped fighting and still studied.

"What is it Y/n?" He asked without taking a glance at your laying form.

Since you were younger than him, you were more frail than Muzan.

"You heard me after all! Why didn't you answer the first time?" Your soft voice spoke out. This had been always a tendency of his. So you wouldn't know if he's listening or not.

He did not move from his spot and continued writing, not bothering to answer your childish question.

Glancing at the maid who stood beside your bed, she nods and bows to you then goes to Muzan.

"Excuse me young first master, but my lady will be resting now" the maid says politely.

He stops his writings and turns to the maid. His red plum eyes stared at the maid's black one's. His frown deepened and his hands trembled as he clenched the pen tightly.

"No, I won't go anywhere. I'm busy studying" he states and turns back to the papers.

The sound of paper and pen was the only thing that made a sound. Both girls stared at Muzan's back.

"But young master, my lady --

"I said no didn't I?" He said his voice turning deep as he glared at her.

The maid flinched at the icy cold glare but sucks it up.

"I'm really sorry first young master. But no matter what you say won't change the fact that you need to leave my lady" she tells your now angry brother. Her voice stern as she was the adult.

"Why you--

"Brother!"

"What?"

You sigh in defeat at how stubborn he could be. You glanced at your maid, you noticed the clenched hands and the biting of cheek. Looking back at your brother's red plum eyes you say...

"Muzan-nii, you need to go. I need to rest--

"No, it's not like I make a lot of noise. You can sleep while I study" he retaliated,

You squint your eyes at him, your eyebrows burrowing as the stubbornness of Muzan was beginning to annoy you.

"Muzan!" You say angrily as you watch him turn your back to you and begin to write.

"No" he says back to you.

You grit your teeth and held in your outburst. He is your older brother after all.

Staying silent as you glare at his back, you breathed out a sigh and turned over to your side and slowly closed your eyes.

Later~~~~~

The scratching of the pen to the paper filled the room. The maid had already excused herself and went out to do her chores.

Muzan looked over all his answers and writings and leaned back to his chair. His legs were numb and so is his wrist that started to hurt. The hours of writing took its toll as his eyes were stinging.

He turned to Y/N's sleeping form, as he hears your soft snores he breathed out a sigh of relief.

The day had ended and moonlight shone through the open window. He never wanted to admit it but... He truly did love his sister. She was the only one who understood the pain as she also experienced the same hardships as him.

The burning hatred for his parents only grew as the didn't care for you. The gifts, love and attention were all for him and not for his dear little sister.

Despite being weak, he studied hard for you. That if the day he comes to receive the title, he was the one to provide for you.

But that all shattered when the doctor said that both of you won't live past 20. You were 2 years younger than him, and the thought of him gone while you suffered made something tug in his heart.

Putting his feet down the wooden floor, he carefully stood up and walked over to your futon. He carefully laid beside you and wrapped his arms around your weak small figure.

"I'll protect you no matter what" he whispered to you. He closes his eyes as y/n snuggled closer to him.

"Brother"

Years passed
Y/n=17 Muzan=20

The day you and Muzan feared has finally arrived. The promenition of your lives only living to the age 20 had arrived.

The doctor had given both of you medicines that he created with the so called blue spider Lily.

When you and Muzan discovered what you have become, you and him had different reactions.

You were scared of yourself while Muzan praised and was happy at the change.

After that, you didn't like what he had become. His personality flipped.

He was more sinister and more easy to anger. Ever since you both had power and skills that no other beings possessed, he had told you that you were both perfect.

"B-brother" you stuttered, he had already scared away the maid and had immediately killed her before she could run.

You stared at his grinning face, the aura of excitement was seen on him.

"I'm telling you Y/n! This is a new chance! We are perfect and powerful!" He claimed happily as he grips both of your hands together, a grin present on his pale face.

"But brother! We cannot just people freely! If they catch us--

You tried to argue. But no matter what you say didn't change his belief.

"Y/n! Listen to me!" He demanded. His grip on your soft hands tightened.

"Do not worry! I will protect you, ok? Just follow me no matter what happens ok?" He requested to you.

Gulping nervously, you let yourself sigh in defeat and agreed.
"Fine"

"Thank you"

He hugs you, your head laid on his shoulder he was now too tall for you and had trouble hugging him back.

You close your eyes and returned the warm hug. It was rare to have these kinds of moments with him, so you had always savoured it.

"I hope you keep that promises brother"