Genshin has a thing for repetition in general, though I admit I don't know much about the overall lore of the world further than the quests we've solved. Samsara cycles and such. Even so, I hold the belief that third time's the charm in most of the quests about breaking these cycles. There is always a third entity that needs to be either hidden, destroyed, or kept, and often is more powerful than the archons themselves, that must be brought to light or pacified in some form (Except... I don't know if it's an exception, but Liyue was weird). Which is to say, yes, as identity goes, the self is two in Teyvat, but that just explains the (dis)order of the world and maybe foreshadows a unification or separation from that other self. (Ei-Makoto-Ishtaroth, Scaramouche-Kunikuzushi-Wanderer, the three moons, Teyvat-Celestia-Abyss, Furina-Foçalors-Neuvillette, etc)

To stay in the doubles dynamic is to remain in Samsara, which is where our traveller is at this time, and why finding the sibling in game stopped being the main quest.

Think about the triad as the knife, and the double as the ego which will inevitably come to an end when the Samsara cycle is transcended. Fun times. More concretely, what I think this means for the siblings, though, is that one of them will not make it out of Teyvat (or die saving it).

At the same time, this is all speculation based exclusively on the archon quests and not in-world exploration or books, etc. I'm, sadly, more of a character-analyzer than a world-theorizer 🤧