“Who was Cain’s wife? Was Cain’s wife his sister?”The Bible does not specifically say who Cain’s wife was. The only possible answer is that Cain’s wife was his sister or niece or great-niece, etc. The Bible does not say how old Cain was when he killed AbelGenesis 4:88 One day Cain suggested to his brother, “Let’s go out into the fields.”[a] And while they were in the field, Cain attacked his brother, Abel, and killed him.Since they were both farmers, they were likely both full-grown adults, possibly with families of their own. Adam and Eve surely had given birth to more children than just Cain and Abel at the time Abel was killed. They definitely had many more children later Genesis 5:44 After the birth of Seth, Adam lived another 800 years, and he had other sons and daughters.
Cain was scared for his own life after he killed Abel Genesis 4:1414 You have banished me from the land and from your presence; you have made me a homeless wanderer. Anyone who finds me will kill me!”This verse indicates that there were likely many other children and perhaps even grandchildren of Adam and Eve already living at that time. Cain’s wife was a daughter or granddaughter of Adam and Eve.Genesis 4:1717 Cain had sexual relations with his wife, and she became pregnant and gave birth to Enoch. Then Cain founded a city, which he named Enoch, after his son.Since Adam and Eve were the first (and only) human beings, their children would have no other choice than to intermarry. God did not forbid inter-family marriage until much later when there were enough people to make intermarriage unnecessary Leviticus 18:6-18Forbidden Sexual Practices
18 Then the Lord said to Moses, 2 “Give the following instructions to the people of Israel. I am the Lord your God. 3 So do not act like the people in Egypt, where you used to live, or like the people of Canaan, where I am taking you. You must not imitate their way of life. 4 You must obey all my regulations and be careful to obey my decrees, for I am the Lord your God. 5 If you obey my decrees and my regulations, you will find life through them. I am the Lord.
6 “You must never have sexual relations with a close relative, for I am the Lord.
7 “Do not violate your father by having sexual relations with your mother. She is your mother; you must not have sexual relations with her.
8 “Do not have sexual relations with any of your father’s wives, for this would violate your father.
9 “Do not have sexual relations with your sister or half sister, whether she is your father’s daughter or your mother’s daughter, whether she was born into your household or someone else’s.
10 “Do not have sexual relations with your granddaughter, whether she is your son’s daughter or your daughter’s daughter, for this would violate yourself.
11 “Do not have sexual relations with your stepsister, the daughter of any of your father’s wives, for she is your sister.
12 “Do not have sexual relations with your father’s sister, for she is your father’s close relative.
13 “Do not have sexual relations with your mother’s sister, for she is your mother’s close relative.
14 “Do not violate your uncle, your father’s brother, by having sexual relations with his wife, for she is your aunt.
15 “Do not have sexual relations with your daughter-in-law; she is your son’s wife, so you must not have sexual relations with her.
16 “Do not have sexual relations with your brother’s wife, for this would violate your brother.
17 “Do not have sexual relations with both a woman and her daughter. And do not take[a] her granddaughter, whether her son’s daughter or her daughter’s daughter, and have sexual relations with her. They are close relatives, and this would be a wicked act.
18 “While your wife is living, do not marry her sister and have sexual relations with her, for they would be rivals.
The reason that incest today often results in genetic abnormalities is that when two people of similar genetics (i.e., a brother and sister) have children together, there is a high risk of their recessive characteristics becoming dominant. When people from different families have children, it is highly unlikely that both parents will carry the same recessive traits. The human genetic code has become increasingly “polluted” over the centuries as genetic defects are multiplied, amplified, and passed down from generation to generation. Adam and Eve did not have any genetic defects, and that enabled them and the first few generations of their descendants to have a far greater quality of health than we do now.
Adam and Eve’s children had few, if any, genetic defects. As a result, it was safe for them to intermarry.