She would have to be another descendant of Adam’s – a sister or cousin to Cain.
Incest? So it seems. We should ask, though: Is incest sin because it reinforces accumulated genetic defects, causing much more likely birth defects? Or is it sin because God says so in Leviticus 18?
That matters — because if we choose to read Genesis as history, then Adam and Eve were created fresh with no genetic defects. It would have taken many, many generations for genetic drift to have got to a place where siblings or cousins risk deformed children when they marry. And the law forbidding incest doesn’t come along until thousands of years later, in Moses’ time.
So if it’s a genetic issue, no problem; by the time Cain marries, Adam and Eve and all their descendants have been happily reproducing without birth defects for decades or centuries.
And if it’s a legalistic issue, still no problem, because that law doesn’t exist yet in Genesis.