She would've had to have been one of his sisters. I don't think Genesis was ever meant to be taken literally, even back whenever it was first written. There are a lot of similarities between it and old Babylonian mythology and others from the Mesopotamian region. Plus, Genesis more than anything in the Old Testament reads like mythology rather than a mixture of myth and history like a lot of the rest does. Later books of the Old Testament read as though they're fantastical and exaggerated stories meant to convey real events in history, like the books of the Kings or Chronicles, but in Genesis you get stories that are crafted to depict the origin of man and the planet, the first lie, the first murder, and the origin of different languages. I think it's silly to take it literally.
It is in Christianity because the whole point of Jesus being crucified and coming back was that he made himself a sacrificial lamb that through his sacrifice washed away the original sin caused by Adam and Eve eating the forbidden fruit. So their existence is vital to Christian theology
