GENSHIN THEORY about who exactly is Caribert and the rest of the story.

So...

(I play as Aether so if i say Lumine somewhere by mistake I do reffer to the other sibling. Thank you, you may read my theories now.)

There's Pierro...
His pupils also look like that.

I've examined the eyes of the three.
Dain is a pure blood. He has a full diamond pupil
Kaeya's look like a 1/4 or 1/3 of Dains. What would mean he's only that partly a Khanri'an. But, why would Dain call him a pure blood? You'd ask.
The curse of immortality is the answer here.
If Caribert did indeed broke it that way, heres few important things it would explain:
Kaeya's aging
The mystery that led Diluc to fury and hatred towards Kaeya (if you have a sharp mind, you'll get it.)
and most importantly - THE OTHER SIBBLING.
Their priotity is to find a way to make Hilichurls turn back into human... Could it be because of Caribert? Do you see how much sense it makes?

Now, back to Kaeya, Caribert and Pierros conection.
Pierros pupils touch with his upper eyelid line, so they are definitely bigger than Kaeyas, but if you look closely, still smaller than dainsleifs. The mask he wears and his clothes at all suggest his Identity.
This is a good justification to the theory that Pierro is indeed Kaeyas father.

Also, I do not think the breaking of the curse would affect Chlotar. He has been devoured, Dain said he's lost his mind and ran away... You could say basing on the words spoken in the quest he was decomposing..
Also even though the curse is called a one of Immortality, it is simply impossible to be fully a mortal or immortal to begin with (this reaches out the game, true. But we need to remember how genshin likes to take on interesting topics from the true life of ours) what means.. He could have simply decomposed in one way or another and lived as a decomposed creature, affected by the curse in the same way it was affecting Dainsleif now. Maybe he died of himself, or Caribert could've killed him to end his suffering... Maybe it was the power that would send all of the hilichurls to sleep. Take a step back to the quests in the chasm, where we first learn about the hilichurls identities.. there was some kind of power that would calm down the ilness.
Chlotars incredible interest in the Abyss Herald and him bowing before the power? Do you get what I'm pointing at now?

It could have been of same kind. That power could have affected him the way it affects hilichurls. Or.. he just went insane and killed himself which is unlikel- although... that gives me yet another suggestion but ill talk about it later. I'll sign it with a: • symbol.

Now a little look at paimon. The symbol on her chest is a reversed sign of reality, and reversions actually appear a lot in genshin.

So the power could have killed him, but given to Caribert by a person of the opposite curse.. could it reverse?

Also THE MASK - is another keyword there.
He has taken off his mask and seen his face, what has been in some kind forbidden for hilichurls. Why? Could it be the way..? for them to break the curse? See.. he took off his mask just before he started transforming somehow. The power seemed to be creeping OUT from under his mask more and more. As if it was LEAVING. He TOOK OFF his mask. Do you get it..? 
THAT could've been the breaking point. + the medicine and power from the Sinner.
(which brings me another theory, and ill mark it with a: × symbol this time.)

So Caribert could have turned human and wandered off on his own.
Now, Caribert could eather be Pierro, or his father. 
Though considering that Pierro is now against the Abyss Order, fighting against the heavenly principales, someones ideas had to drive him to that.. It could have been Chlotar. Pierro being Caribert would make just as much sense as he would know the other sibling, the theories of Chlothars - the Abyss Orders founder. He could by a mind of his own dissagreed with that at some point. Learnt the thruth of this world and joined the fatui or.. Even could have been an cofournder of it toghether with Tsaritsa.
Also the whole theory would be explaning "The Loom of Fate"
as the hope, and proof, that it is possible to turn hillichurls back into humans...
That's what Lumine would fight for, as she has witnessed it. That is how exatly the Sinner...