The twins initially fall to Teyvat while traveling between various worlds after their home world was destroyed. They crash-land and fall unconscious from their "meteorite" impact.
The Traveler's Sibling is the first to awaken, and leaves the yet unconscious Traveler to temporarily explore more of the world that they landed on. The sibling then at some point witnesses the Cataclysm and the Gods destroying Khaenri'ah. They rush back to their impact-point and wake up The Traveler, showing them the destruction that's being wrought.
During the beginning cutscene in Genshin Impact, both siblings are fighting against the Unknown God. Depending on which gender the player chooses, they will have a name attached to them and will become the Traveler. The other sibling (Lumine or Aether) will then be attacked by the Unknown God and sent away in the form of a cube. The Traveler will then continue to fight before being subdued and having their powers sealed away, sent to wander the land until they come across Paimon.
After which, at some point after the Cataclysm, the Traveler's sibling wakes up, and travels the world of Teyvat with Dainsleif. Sometime at or near the end of their journey, the sibling realizes the "truth of this world" and also subsequently, the truth behind the origins of Dainsleif, and parts ways with him.
During the Archon Quest, Prologue Chapter - Act III, intelligence passed on from Kaeya and background history from Venti reveals that the Abyss Order has recently gained a new leader named the Prince or Princess. At this point, while the Traveler does not know who this leader is, the player can conclude that the Prince or Princess is the Traveler's sibling.
In the story teaser trailer We Will Be Reunited, the Traveler's sibling is depicted as Lumine. In it, she is shown traveling through Mondstadt carried by a Ruin Guard and is revealed to be behind the plot of the Abyss Mages to weaponize the dragon Dvalin. An Abyss Mage refers to her as "Your Highness" and proceeds to tell her about their foiled plan — she is shocked to learn from the Mage that her brother had a major role in foiling that plot. Later, she runs to a cliffside, where she witnesses her brother fight hilichurls in the grounds of Stormterror's Lair unbeknownst to her brother. This same area is revisited by the Traveler and Dainsleif during the quest Bough Keeper: Dainsleif wherein the Traveler senses the previous presence of their sibling through the dandelion they stepped on previously. There, Dainsleif finds a Ruin Guard's footprint nearby while also speaking vaguely about a "traveling companion" he had been with before. It is later revealed that the sibling was Dainsleif's travel partner, but due to currently unknown circumstances, they did not reach the end together.
In the Archon Quest, Chapter I - Act IV: We Will Be Reunited, the Traveler's sibling briefly meets with the Traveler in front of the Defiled Statue after an altercation between the Traveler and an Abyss Herald. There, the sibling asks the Traveler to not stop them or the Abyss. The Traveler's sibling also reveals the truth of Dainsleif's origins and that the monsters of the Abyss were formerly Khaenri'ah's people before being transformed. The Traveler tries to persuade them to continue with their original plan, which the sibling agrees with, but after their war against the divine is over. The sibling then tells them to continue traveling Teyvat in the hopes of seeing the true nature of the world presided by the Seven and Celestia before leaving with the Herald through a portal.
At some point, the sibling visited The Chasm and saw dead hilichurls, who had gone inside to die in peace due to a water pool receding the curse's effects. Angered and upset by their needless suffering, they authorized the activation of a device that would supposedly amplify the water's effects to the point that the curse would be removed for good regardless of the success rate. If the device worked, the hilichurls would be reverted to their previous forms as humans of Khaenri'ah, but if it failed, it would inflict extreme, needless pain on the hilichurls. As expected, the device backfired before it was destroyed by Halfdan and the plan stopped by Dainsleif and the Traveler.
At some point in the past, the Aranara race of Sumeru were visited by a human they called "Nara Varuna." The Aranara mention that this person was similar to the Traveler, frequently mentioned their sibling, and their pronouns change to be the opposite of the Traveler's, making it reasonable to assume Nara Varuna was the sibling. "Nara Varuna" was friendly to the Aranara and gained that name by helping with the Varuna Contraption. Before parting with the Aranara, "Varuna" told Araja of another golden-haired human who would visit them in the future and give them even better memories.
In the Archon Quest, Chapter III - Act V: Akasha Pulses, The Kalpa Flame Rises, new snippets of knowledge were shared between the Traveler and Nahida. The latter disclosed the information about the Traveler's identity as a Descender of the world of Teyvat, while the Sibling is not by contrast. This conclusion was said to be from a presence of information gathered in the Irminsul records about the Sibling, which is told to be impossible by Nahida unless if they (the Sibling) are naturally apart of the world to begin with. It is currently unknown as to why this is.