The reason Abyss Sibling Isn't a Descender:
Okay so we know that what makes a descender isnt that they come from outside Teyvat, but that they have a will to "rival a world." And I think it mentioned something in some skirk related lore, maybe her weapon I dont remember, that if one loses their world but perseveres, they can gain the will of a descender. Perhaps the Abyss Sibling lost their descender status because they lost their memories, and thus, the will they had cultivated after losing their homeworld.
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Also Voyager comes to Teyvat 2 times. Once long before Nibelung departs from Teyvat and for a second time after HP comes. But becuse both of these happened at least 10k years ago that would require our spaceship to come to Teyvat in total of 3 times (2 times in antiquity and 1 more time 500 years ago). But from the latest quest we know it happened only 2 times (1 time in antiquity and 1 more 500 years ago).
But yes, the ship having to fly by three times not two would disprove this… unless a third time is listed in the corrupted files MC mentioned
About the corrupted files i agree with you. If one of the fly bys in there it wouldn't create a contradiction but it is kind of impossible to say if they are in there or not because data is gone so best thing to do for now is to stick with what we know for sure.
How reliable our sources really? Did they call her a she originally because they used their female form when they interacted with the people on Teyvat? Maybe their previous body was also acquired by projecting their mind into it. Maybe they are androgynous and can change their form at will and a female form just happened to be the one they used at the time. One of the key pillars of Genshin lore is that the sources are not really all that reliable especially because all sources have eith some kind of flaw, bias or both.
I am not really well versed in the lore related to Voyager. So if you have actual answers to the questions that i posed just know i would like to know them. They are not purely rhetorical.
I am so hyped for the next Dain's quest to get more information about Abyss Sibling. I like their character so much, this broken, dramatic anti-hero with a pure heart, hindered by guilt, who has to visibly battle their emotions when Traveler is around to avoid giving up on their cause, which there twin cannot understand just yet to the full extent.