What Genesis says
After Cain killed Abel, God sent him away, and Cain settled in the land of Nod, east of Eden (Genesis 4:16). The next verse says that Cain was with his wife and that she gave birth to their son, Enoch (Genesis 4:17). Her name, parents, and the circumstances of their marriage are not recorded.
So the most direct answer is simple: Cain’s wife is unknown by name. Genesis tells readers enough to show that Cain had a family, but it does not provide every family detail from the earliest generations.
And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
Why she was likely a close relative
Genesis describes Eve as the mother of all living (Genesis 3:20). It later adds that Adam had sons and daughters besides those whose names are given in the story (Genesis 5:4). Taken together, these statements are why many Christians conclude that Cain’s wife came from Adam and Eve’s wider family.
The Bible does not specify whether she was Cain’s sister, niece, or another close relative. It also does not give a complete timeline for how long Adam and Eve’s family had been growing before these events. It is reasonable to draw the usual conclusion, while being honest about what the text does not say.
And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:
Did Cain meet her in Nod?
Nod is named as the place where Cain lived after leaving the Lord’s presence (Genesis 4:16). But the passage does not say that Nod was home to a separate people group, or that Cain found his wife there. A place name alone does not explain where she came from.
Cain feared that someone who found him might kill him (Genesis 4:14). This concern may fit with a family that had already grown beyond the people specifically named in the account. Still, Genesis does not explicitly connect the people Cain feared with his wife, so that detail should not be treated as certain.
And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden.
What about later marriage laws?
Much later, God gave Israel clear commands forbidding sexual relationships between close relatives, including siblings (Leviticus 18:9). Those instructions belong to God’s law for Israel and should be taken seriously.
Genesis does not pause to explain every difference between the first human family and the later laws given through Moses. Its main point in Cain’s story is the spread of sin, God’s justice, and God’s continuing care. Reading Genesis 3 to 5 for yourself can help keep the clear facts and the unanswered questions in their proper place.
The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover.