Undertale AU Sibling Oneshots

Frisk x Twin! Sick! Reader

(y'all on the surface)
(And because I can, Flowey is like an adopted sibling to you to)
(And just cause if you want, you can look like Frisk or Frisk can look like you!~)

You walked alongside Frisk, hand in hand, from school.
You and Flowey listened to Frisk chatter on about their day.
"—and then Matthew mixed me up with you, (Y/n)!"
"Was Flowey in the school yard?"
"Yep! Flowey wasn't with me so he thought I was you. He was asking about science notes."
"Oh yeah, I borrowed those..." You hummed, glancing up, "He's not getting them back."
Flowey, from Frisk's backpack, busted out laughing.
"(Y/n)!" Frisk laughed a little, "That's mean!"
"He was mean to you last Tuesday! So he's not getting his notes back!"
"Sounds fair to me!" Flowey chimed in, causing Frisk to sigh.
"You two can be so mean sometimes!"
"Love you too, Frisk!" You grinned as you opened the door, calling out.
"Momma, we're home!"
"Hello, my children!" Toriel called from the kitchen, the smell of a cooking pie following her voice, "How was school?"
"It was good!" You chirped, taking Flowey from Frisk to move him to his favorite pot.
"(Y/n) won't give a kid his notes back because he was mean to me!"
"Tattletale..." You and Flowey both grumbled in unison.
"Well," Toriel sighed as she came out of the kitchen, (favorite flavor) juice in hand, "I can't say I don't see their reasoning for that one."
Toriel patted your heads after giving you both your juice. "Just make sure you return them, (Y/n)."
You puffed out your cheeks as Frisk giggled. Toriel went back into the kitchen, and you, Frisk and Flowey went back to your room to work on homework.
Or well, you and Frisk did, Flowey got to play video games.

School was the usual, you handed Matthew's notes back, you and Frisk went through your usual schedules, everything was normal.
Except when you didn't feel so good once free period rolled around.
Instead of going and hanging out with the other kids, you went to sit in the shade of a tree, feeling like you were on fire.
"(Y/n)?"
You response to hearing Frisk's voice was to groan, not having the energy to lift your head.
You hear the rustle of dirt and grass as Flowey dug into the dirt at your side, Frisk's hand finding your forehead a moment later.
Frisk quickly pulled their hand back.
"You're burning up!" They exclaimed softly.
"Wanna go home..." You mumbled almost pitifully, blinking blearily and grimacing.
"I'll go get a teacher!" You listened as Frisk's footsteps faded away, feeling Flowey's vines wrap around your wrist for comfort.
You offered a small, brief smile to him, groaning again as a teacher walked over and felt your forehead.
"I'll call Toriel to come take you home, alright?"
"Okay..." You rasped.

You buried your face into Toriel's shoulder as she signed you and Frisk out, thankful that she had thought to use magic to make her feel cool for you.
In an attempt to have Frisk not catch your cold, Frisk rode home in the front seat, but that didn't stop them from glancing back at your flushed, tired face every five seconds.
Time was a little bit of a blur. You blinked, in the car, blink again, being carried in through the front door, another blink, suddenly in your bed.
Toriel had you change into some clean, lighter clothing, taking your temperature afterwards.
"102..." she hummed, frowning a bit before offering you a soft smile. "I'll get you a cold rag for your head. If anything else starts to come up, let me know, alright?"
You nodded, taking the glass of water she offered, drinking almost greedily for a moment.
"Will Frisk have to sleep in a different room?" You mumbled, letting her set the glass on your nightstand.
"I'm afraid so, little one."
You frowned at that, but laid down as she instructed.
You fell asleep just as you felt the soothing chill of a rag on your forehead.

You awoke feeling like you were dying in the middle of a desert. Trying to keep your nerves in check, you carefully rolled over, reaching for the water that was left on your nightstand.
It was room temperature, but it was cooler than the rest of your body, so it was practically a saving grace at that moment.
Stopping yourself from flat out chugging it, you set the glass down, shaking a bit as you tried to catch your breath.
The clock read about four in the morning, making you inwardly groan.
You'd head to the bathroom, come back, flip literally everything on your bed to the cool side, and then go back to bed.
That was the plan anyway.
You made it to the bathroom, but on your way back you overheated and had to slump to the floor.
Any further and you'd probably make yourself sicker.
Just as you were debating on crawling back, Frisk showed up next to you, carefully tugging you to your feet and back to your room.
"You're supposed to be asleep, Frisk!" You whisper-yelled at them.
"So are you!" They whisper-yelled back.
Once your bedsheets and pillows were flipped, and you were settled, Frisk slipped away to get you cold water and a new rag.
"I don't want you getting sick, Frisk." You mumbled as they handed you the water.
"I'll be fine!"
You made a face at them, your expression melting into one of relief as the cold liquid rushed down your throat. You leaned back after setting the glass to the side, Frisk placing the rag on your forehead.
You then watched as Frisk walked over to the closet, quietly tugging out a floor-fan.
"Forgot we had that."
Frisk giggled quietly at you, plugging it in and facing it to you.
"Frisk? (Y/n)?"
You both perked up to Toriel's voice in the doorway, a curious yet stern expression on her face. "Why are you two awake?"
"(Y/n) woke up and couldn't make it back to their room, so I helped them out a bit."
Toriel's expression softened, and she reached down to ruffle Frisk's hair. "Thank you, child, now go back to bed, it's late."
Frisk nodded, waving to you before slipping out back down the hall.
Toriel then walked up to you, sitting next to you and humming softly as she stroked your hair.
In no time at all you were asleep.

You awoke to Flowey staring down at you.
"Good, you're awake. Mom made breakfast."
Your stomach growled at the words, and after a few minutes of helping Flowey into his rolling pot, you trundled down the hall with him clinging to your wrist.
You didn't feel nearly as horrible as last night, but you could tell you still had a fever.
"Good morning, (Y/n). Good morning, Flowey."
You gave a soft, tired hum, and Flowey gave his usual halfhearted reply as a response.
Frisk waved from the table, mouth already full of pancakes.
You let Flowey roll over to Frisk, sitting down across the table as Toriel brought you something light for your stomach before feeling your forehead and cheeks.
"Mm, your fevers almost gone. Another day of rest and you should be fine."
You gave a tired smile and watched her disappear into the kitchen.
"Hey Frisk."
"Hm?"
"How'd you get the better immune system between the two of us?"
"Because I don't keep other people's notes for longer than I should."
"Hey!"
Toriel busted out laughing from the kitchen.