Who was Cain's wife?

The Bible does not specifically say who Cain’s wife was. The only possible answer was 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 Abel (Genesis 4:8). Since they were both farmers, they were probably both full grown adults, and maybe with families of their own. Adam and Eve had surely had more children than just Cain and Abel at the time Abel was killed and they definitely had many more children later as said in, Genesis 5:4. The fact that Cain was scared for his own life after he killed Abel indicates that there were likely many other children and perhaps even grandchildren or great grandchildren of Adam and Eve at that time. Cain's wife would've been a daughter or granddaughter of Adam and Eve.

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 was enough people that intermarriage was not necessary which is said in Leviticus 18:6-18. It may seem strange or even disgusting to think of Cain's wife being his sister. In the beginning, since God started with one man and one woman, the second generation would have no choice but to intermarry amongst themselves.

God bless.