>>531146010
Simple, it means the traveler's sibling is the Heavenly Principles currently resting in the sea of flowers at the end, they are not the primordial one but their replacement that took over after the harbingers burned the old world away on a previous iteration of teyvat causing the destruction of the world with new Khaenri'ah being the last surviving place where they started a new life and eventually repeated the Khenri'ah Cataclysm where Asmoday swooped in to stop the siblings from leaving and realized the sibling still had memories of the old world that could be used to restore the samsara dream of teyvat, and used them as the new host (Heavenly Principles). The sibling saw their own sibling (the traveler) at the end of their own journey, also sleeping in the sea of flowers at the end, and believes them to still be the current heavenly principles in the current iteration (which is wrong, since it's them this time) but they cannot tell the traveler because learning the true nature of the dream before one is prepared can break one's mind (as noted by Nahida in the subseruz samsara). The events of the past will be repeated, the harbingers will burn the old world again, and the traveler will ascend as the new heavenly principles after meeting Asmoday again.