Upside down

Chapter 1

This time your brother didn't say anything but just crossed his arms and glared at you, asking for an explanation
You put your hands in your pockets, shrug your shoulders and say: "In short, we're in a different universe with uh other versions of us. The multiverse theory really does exist, man"

In front of the other Sans and Papyrus, you look natural, and it's surprising how well your acting skills work even though you're not an actor and you really need some private space so you both can talk now about how the fictional characters are in front of you or about how the two of you woke up in the bodies of two creatures created by both of you. Could this be what they call isekai? Although your memory of your world is a bit fuzzy, the last thing you see is definitely not a truck that hit you

'Haiz, I need to get out of this situation now to know where I am. Hope my idiot brother will act as natural as possible '

Luckily, your brother didn't seem to panic. He gets up, ignores the arm you're extending and looks at the others saying, "Sorry about that, I'm a bit surprised and a bit hot."
"SORRY ABOUT THAT, I WAS A BIT SURPRISED AND LOST MY TEMPER EARLIER"

Speaking of other people, you noticed that they look at you strangely, is it because of your actions just now? Probably because Sans and Papyrus in every AUs always care about each other, maybe your wake-up action was a bit aggressive

"IT'S OK OTHER ME, WHY DON'T WE GO TO THE LIVING-ROOM TO TALK"
Papyrus said, showing an awkward smile

You know you're the one who started this dilemma, so you don't want to embarrass the landlord. It's really not appropriate to talk in that room anyway
"Sure. Please,lead the way"

And so, everyone left the room (probably the basement) of the house... No, it's more of a mansion though it's certainly not as big as the ones in the movies
You look out the window and you know you're in a big house...in the woods
'Looking at the open space, I'm sure this house has a yard...or at least a backyard'

Arriving, everyone sat down together, luckily there was more than one sofa, who knows how many skeletons are in this house

"Short story or long story?"
Sans said, right to the point

You ponder for a bit and then say: "I think it's better to explain from the beginning because we are also quite surprised that you guys live in uh....a big house like this"

"Ok"
___________________
In short, at first only Sans and Papyrus lived together after the monster returned to the surface. They didn't originally live in a house this big, at least before Sans started trying to fix the machine in their basement-the one they got from the basement of their house in Snowdin
It was a big mistake
The machine spontaneously explodes and proves the multiverse theory exists by carrying two pairs of Sans and Papyrus. 1 pair looks like their swapped version, 1 pair looks like a more violent version of them

It was very chaotic at the time, from the endless questions to the fights caused by disagreement, mainly starting with the more violent version. Sans and Papyrus have tried to help them from common to personal issues like the complicated brotherhood of the violent version or Papyrus' overprotectiveness with his big brother from the swapped version

Sans and Papyrus even confronted and explained to their friends during one of their visits. Of course, over time, things gradually stabilized and Sans, US!Papyrus and UF!Sans worked together to repair the machine to send everyone back to their universe (although at the time UF!Papyrus was a bit annoyed when he known that his brother can fix things related to machines and science)

And another explosion appeared, two more pairs appeared. A couple with Sans looking like HIM made Sans, US! Papyrus and UF! Sans takes a long time to get used to his face and the other pair which that Papyrus is no different from the original Papyrus in terms of personality but his Sans is quite funny, he is a royal scientist and likes to say "Bruh"...a lot

Because the number of skeletons is increasing day by day, living in an ordinary house is quite cramped. Their friends after knowing this, are willing to help them find a house big enough to prevent another explosion. Especially Toriel when she actively provides them with cash. This caused both Sans and Papyrus to refuse together because ever since humans and monsters started living together, the gold that the monsters used was exchanged for money, so now they are quite uh.....rich (lol)

But of course that didn't work out because Toriel didn't accept it at all, who would dare refuse the Former Queen of Monsters a second time

After moving into this house, everyone started to work together to fix the machine, thanks to the help of a real scientist, the machine did not explode for a longer time than usual. Just when everyone thought the machine was about to be fixef anytime soon. This morning, around the time they were doing their work, without even touching the machine, it exploded again.
This explosion was completely spontaneous, the people in the house immediately ran down to the basement and saw you and your brother
________________
"And that the story" Sans said

This time neither of you said anything, your younger brother was thoughtfully drinking a cup of tea prepared by US! Sans, and you didn't move, putting your hand on your chin thinking about the information just now
'So we're in the timeline-no, rather, after Undertale's Pacific round. According to Sans, I'm sure except Underswap, Underfell, the other two AUs are probably Echotale and Epictale'

This can't get any worst

Seeing that the two of you didn't say anything, US!Sans tried to calm you two down
"I KNOW IT'S A BIT CONFUSING BUT DON'T WORRY, THE MACHINE WILL BE FIX AND WE WILL BE HOME TOGETHER. OH I KNOW! LET THINK ABOUT YOUR NICKNAME!!"

"NICKNAME?" your brother said

"EXACTLY, BECAUSE OF AVOID CONFIDENTIALITY WE HAVE CREATED OUR OWN NAME! LIKE ME, THE GREAT PAPS AND MY BROTHER CLASSIC!!", Papyrus, or rather Paps proudly introduced. Others also started introducing their own nicknames

US! Sans is next: "I'M THE MAGNIFICENT BLUE AND THE ONE WITH THE ORANGE HOODIE IS MY BROTHER STRETCH!!!"
Blue said while going to Stretch and gave him a hug

UF!Sans has to introduce himself as if he's bored of doing this
"I'm Red, my brother is Edge but he's not here right now"

"Finally it's my turn bruh"
ET! Sans suddenly said, walked in front of you, held out his hand and said: "The name's is Epic, nice to meet you new bruh"

Epictale is one of your favorite AUs (second place) even you wouldn't have thought of meeting your idol in real life like this. You can feel your brother's mocking gaze but you don't care
'Must be calm!!!!'