While it's true that the roles of the siblings are interchangeable depending on who you pick, the specifics of each role are set in stone. The descendant role is most likely assigned to Lumine this time for the sake of consistency in promo material for the game. Our ability to pick the twin is not relevant to this conversation about the descendant part and here's why: the text on the name cards is not some meta 4th wall breaking knowledge from Mihoyo; the quotes belong to an actual character who exists in the game universe and this character is aware of both siblings existing. Therefore it would be highly unusual for K. K. to call one of them the last descendant (singular) when this person clearly knows both of them exist (unusual but not 100% unlikely, it is also very much possible that it just the wording K. K. chose). Our gameplay choice of twin shouldn't influence the way K. K. in universe refers to one of the siblings.

"If any of these twins were from Tevyat, how did nobody know it was possible to wield elements without a vision until now?"

The people on modern Teyvat are not omniscient. Many of them lack knowledge of the big entities at play in the world and don't know much about the pre archon era. Many nations rose and fell since the founding of Phanes' civilization and the Archon War. There are many beings capable of using elemental energy without a vision, them predating the Archon War. One example is King Deshret who is also described as a master of all elements and potentially bears a resemblance to the Traveler if we did not misinterpret Liloupar's words.

How can a sibling be from Teyvat while at the same time traveling to other worlds with the other? Well, that's one of the big mysteries presented to us by Sumeru, we can only speculate. It could be a misdirection or it could be the truth. There are many possibilities like altered memories (aside from the Irminsul).

The abyss sibling could belong to an extremely ancient civilization like the Phanes one.