Cain married one of his sisters that were born to Adam and Eve. One of Adam’s daughters could have become Cain’s wife as there was no law that condemned such relationships as incest until later.

Marriage between a brother and a sister (though not the sibling) is still in vogue in some cultures. For instance, Abraham had married Sarah, his sister. (Brahmin wives address their husbands as “anna” – brother! Nairs address them as “Yettan” imitating the Aryans. Thamizhar address them as Maamaa).

Incest was declared a sin by God when He gave His laws through Moses to His people because of the gene mutations that had given rise to sub-normal malignant human features by then.