Where Did Adams Sons Find Wives
Adam and Eve where told to multiply upon the earth: "And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth,
..." (Genesis 1:28)
Apparently Adam’s sons found wives from among their sisters.
"After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters."
(Genesis 5:4)
The Jewish historian Josephus wrote “The number of Adam’s children, as says the old tradition, was thirty-three sons and twenty-three daughters.” Antiquities of the Jews section 67:4
So who was Cain’s wife? His sister.
"And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch."
(Genesis 4:16-17)
Cain was married before he went to the land of Nod, it must have been a sister.
"And Cain said unto the LORD, My punishment is greater than I can bear. Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him."
(Genesis 4:13-15)
So who was Cain afraid of? The many descendants of Adam and Eve.
In the beginning it was normal in an effort to
populate the earth, and was not banned until we read it in Leviticus.
Leviticus 18:6-18 “None of you shall approach to any that is near of kin to him, to uncover their nakedness: I am the LORD. The nakedness of thy father, or the nakedness of thy mother, shalt thou not uncover: she is thy mother; thou shalt not uncover her nakedness. The nakedness of thy father's wife shalt thou not uncover: it is thy father's nakedness. The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover. The nakedness of thy son's daughter, or of thy daughter's daughter, even their nakedness thou shalt not uncover: for theirs is thine own nakedness. The nakedness of thy father's wife's daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness. Thou shalt not uncover the nakedness of thy father's sister: she is thy father's near kinswoman. Thou shalt not uncover the nakedness of thy mother's sister: for she is thy mother's near kinswoman. Thou shalt not uncover the nakedness of thy father's brother, thou shalt not approach to his wife: she is thine aunt. Thou shalt not uncover the nakedness of thy daughter in law: she is thy son's wife; thou shalt not uncover her nakedness. Thou shalt not uncover the nakedness of thy brother's wife: it is thy brother's nakedness. Thou shalt not uncover the nakedness of a woman and her daughter, neither shalt thou take her son's daughter, or her daughter's daughter, to uncover her nakedness; for they are her near kinswomen: it is wickedness. Neither shalt thou take a wife to her sister, to vex her, to uncover her nakedness, beside the other in her life time.“
Leviticus 20:17 “And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness; it is a wicked thing; and they shall be cut off in the sight of their people: he hath uncovered his sister's nakedness; he shall bear his iniquity.“
The likely reason for banning inter family marriage was most likely to decrease the chances of incest with children, and mutations causing diseases and deformities in future generations.