I'm not in the mood to write it, so have a video from my YouTube channel:

https://youtu.be/ZcS2SwK5jVs

https://www.youtube.com/watch?v=ZcS2SwK5jVs

And here's the description of my video:

Characters:
X!Gaster (X)
Handplates!Gaster (Handplates)
Science!Sans (Sci)
Glitchtale!Gaster (G!G; Gaster)
Handplates!Papyrus (Papyrus)

Which character is which?
The guy with the cape is X!Gaster (bad design ik), the one in the lab coat is Handplates!Gaster, Glitchtale!Gaster is the one in the grey coat and you most likely figured out the rest.

What does X!Gaster mean with "AU system"?
Basically in my AU, each Universe has a code, which is kinda like a script for the AU. If something happens that isn't SUPPOSE to, it's considered as a glitch or bug. The AU then either resets a small part of that timeline, leaves it unchanged if it's a very small glitch with barely any effect, or completely deletes it.

Refering to the answer above, why didn't Sci get deleted, and how did X!Gaster know about the AU system?
(Why is the answer so complicated...)
Ok, so AUs can detect if there are big glitches in other AUs. They can fix each other, but only with the monsters or the creators. AU codes can't make desicions for a character, but if it's REALLY needed, they'll do it. X!Gaster knows about AU codes, because he read a small (but complex) section about it in a book about AUs. The AU code made him decide to give Sci to his friend Handplates. Handplates has a bug INSIDE him, so he can only create 1 skeleton (Papyrus). He's overall weaker than the original Handplates!Gaster.
So the two glitches are: Handplates can only create one monster and X has one extra child. Those two bugs match up perfectly, and that's why Sci didn't get deleted.

In the video, Sci saw his "brother" get killed. Why didn't he seem sad in the following parts until he grew up (1:34) ?
Well, it's a small detail I noticed AFTER i exported the video, so basically a malfunction of my brain. Let's just say, Sci is practically a god in hiding his emotions.

Where are they now? (X, G!G, Sci)
In (my) Omega timeline! X stopped looking for an AU, but overwrote all AU codes (not a lot, but some parts) and is the principal of a university and a part-time scientist. Gaster lives with X and works as a teacher at X's university and as a scientist. Sci is 21, lives with X and Gaster, studies at X's university and sometimes works as Gaster's assistant.
(What's funny is that X and Gaster aren't married (yet!), not even dating, even though they live together and raise the same child.)

So, who exactly are Sci's permanent family members now?
X, Cross, and X!Papyrus. There are some who aren't "blood-related" to Sci. Gaster, as his father and Error, as his brother (they call themselves brothers, Error doesn't live with Sci). There are some some more people joining the family in the future (related and non-related), but I won't tell you now.

Those are all the question that came in my mind, if you have one I didn't answer, feel free to ask it!

Bye!