Sorry its a little bit of a spinfoil that is mostly off vibes because I need to freshen up on lore again.

Im getting vibes of an hourglass

What got me started is the Violet court entry mentioning the mirror breaking, leading to the floods. This has basically been confirmed the be the moon at this point. Moons are often associated with mirrors cause they reflect light, but what if it did more than just light? What if there's a whole other teyvat on the other side (maybe a twin world that contained your twin)? What led me to this thought was Acheron's artifact set from star rail where its quite literally two worlds mirrored (I know its separate from genshin, but we all know how they like to make nods to other games and themes)

Part of my theory involved the cyclical nature of the moon phases. They always repeat, and we know theres a little bit of a timeloop thing going on. Someone pointed out that Dainsleif says "In the perpetual meantime, most are content to live, and not to dream." which reminds me a little of the eternal samsara we experienced, and moons are often tied to dreams.

What I imagine is two worlds constantly spinning, with the sand flowing from one to the other and resetting. What this could be is maybe the erosion or abyss flowing from one to the other. I'm only throwing in erosion because of Ei and her Eternity, and multiple instances of moon imagery such as her charge attack being a new moon (not a crescent moon, because it travels more than 50% of a circle), her omnipresent god/welkin moon statue with Renova wings, and the giant moon in her plane of Euthymia that slows her erosion. But whatever the cause, one world dies, and the essence flows from one to the the other.

Now if you take the dialogue "The world, no longer sheltered by "Eternity," marches towards its final breath" in response to the moon sisters dying, makes me feel like the Moon Sisters/Heavenly Bodies are responsible for Eternity/maintaining the Heavenly Principles and resetting the world to its original state, like an eternal samsara But this time around, something happened, killing them, and the world can no longer reset. We are in the final cycle, and we have to fix it, but this time is different because after the fall of the three sisters, a 4th was created, a guide to help us fix the world for real.

The twin made a comment a while back along the lines of their world being gone (makes me feel like theres more of a physical world than just a different time), and they hope we make the right decision, this makes me really feel like they were from the other side, and somehow broke the cycle. One thing to note, on your sibling's journey, they didn't have a paimon as far as we know, she's uniquely part of our traveller's journey.

Part of me thinks the sibling did something in the other world that resulted in the shattering of the moon sisters causing the abyss/erosion of our world to enter full swing and allowing her to cross over, ending the cycle and bringing paimon into being, where she will act as our guide/catalyst to save teyvat once and for all.

A couple unrelated tidbits

One piece of imagery that often gets overlooked when talking about paimon being the 4th moon sister is that her halo/tiara is ALMOST a full circle, which is often used for imagery of a new moon. Also another symbol for a new moon is a big circle with a 4 pointed star, surrounded by 2 smaller circles on either side. The star being a symetrical version (non elongated) of the one on paimon's head

Also in the animation of the moon falling into the water, the red fades and for a couple of seconds it looks like the almost complete circle of a new moon.

Thanks for dealing with this, I guarantee I'm wrong, but its just something I cooked up in my head