Where Did Cain Get His Wife?
Adam was the “first man” (I Cor. 15:45). Eve was “the mother of all living” (Genesis 3:20). There was no pre-Adamic race. Cain was the first son of Adam and Eve (Genesis 4:1). After he killed his brother Able he was fearful “others” might kill him (Genesis 4:14). It is said had a wife (Genesis 4:17).
From where did these “others” come? Genesis 5:4 says of Adam “he begat sons and daughters.” Adam and Eve had many children.
Later as the race developed for genetic reasons interfamily marriages were forbidden. At this early stage the gene pool was such that to initiate the expansion of the human race brothers and sisters married. There is no other explanation.
Adam lived to be 930 (Genesis 5:4). It is likely some of the offsprings were of such diverse ages that they were not reared in the same family setting.