1 Corinthians 15:45 (NKJV)
45 And so it is written, “The first man Adam became a living being.” The last Adam became a life-giving spirit.
if Adam was the 1st Man, and Eve the 1st woman from Adam(Eve was made from Adam’s side (Genesis 2:21–24)—this was a unique event. ), and God told them to Multiply and fill the earth,......then i am sure he (Cain) either married his Sister or Niece, ....... either way, a brother and sister would have married in the beginning.) The Hebrew word for “wife” used in Genesis 4:17 (the first mention of Cain’s wife) is ishshah, and it means “woman/wife/female.”
And Cain knew his wife [ishshah], and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son—Enoch (Genesis 4:17).
The word ishshah is the word for “woman,” and it means “from man.” It is a derivation of the Hebrew words ‘iysh (pronounced: eesh) and enowsh, which both mean “man.” This can be seen in Genesis 2:23 where the name “woman” (ishshah) is given to one who came from Adam.
And Adam said: “This is now bone of my bones and flesh of my flesh; She shall be called Woman [ishshah], because she was taken out of Man [iysh]” (Genesis 2:23).
Thus, Cain’s wife is a descendant of Adam/man. Therefore, she had to be his sister (or possibly niece).