Who was Cain's wife?
Cain was Adam and Eve's first son [Genesis 4:1] and as he grew it became evident to his father that Cain's interests were more earthly than heavenly. Adam, the priest of the family, gave Cain the task of "tilling the ground" to provide vegetables for the only human beings on earth. In the meantime, a second son, Abel, had been born, and he was given the task of looking after his father's sheep. The sheep – or goats, since they are related – were not used for food. They were used for the atonement sacrifice made by their father, while their skins were used for making clothes. Sheep or goat's milk may have been used to make cheese, but certainly God had not yet given permission for man to eat meat. Cain became jealous of Abel, murdered him in the field and was confronted by God. Cain denied the crime, God cursed him and then sent him into exile to live in the land of Nod [Genesis 4:3-16]. The very next verse [v.17] declares, "And Cain knew his wife, and she conceived and bore Enoch. And he built a city …"
To find out who was Cain's wife, we begin with Scripture as the primary source, then search the non-inspired apocryphal literature for other clues and make a judgment. In those early days man lived almost a thousand years (e.g., Adam lived to 930 years [Genesis 5:5]). Consequently, man matured much more slowly as we see from the age of the fathers at the birth of their first son. Adam and Eve were, of course, the same age and, according to The Book of Jubilees, Eve gave birth to Cain in her late sixties [say, 70], then Abel was born seven years later [Eve 77]. We see from Genesis 5:3 that Adam was 130 years old when Seth was born, leaving 130 minus 77 = 53 years between Abel and Seth for other children to be born. The data in The Book of Jubilees shows that Eve had one birth every seven years during her period of fecundity, so that she could have had seven other children [53 divided by 7 = 7] other children between Abel and Seth. The fact that this was not mentioned and their names not given in Scripture indicates that they were probably females. So it is entirely possible that Cain, now about sixty, would have chosen his nearest sister who would have been 130 minus [77 + 7] = 46 years old and not yet mature. According to The Book of Jubilees, her name was Awan meaning "iniquity."
Reference: Charles, Robert Henry [translator]. 2005. The Book of Jubilees or the Little Genesis. Original publishers: Society for Promoting Christian Knowledge, UK. Published 2005 by: Ibis Press, Berwick, Maine.