With only Adam and Eve as the only parents of all humanity, Cain and Abel, then, must have married their sisters. All of the first-generation siblings married each other in order to populate the earth. At that time there was no law against incest. Because the genetic line was so pure in the beginning, there was no problem with inbreeding. But as the population grew large enough, and as the risk of genetic problems increased due to sin’s curse, God outlawed marriage between immediate family members (see Lev. 18:6–18).
Genesis 4:17 And Cain knew his wife, and she conceived and bore Ēnoch. And he built a city, and called the name of the city after the name of his son—Ēnoch.