Where does it say that?
The book of Enoch tells us that Cain took his sister as his wife.
Where? I only expressly say "Pentateuch" explicitly to avoid all the "Oral Torah is Torah too" debates that would undoubtedly arise. Is that a problem for you?
The good and the bad seed. One hears one doesn't.
Kenite
Strongs 7017 Hebrew.
patron from 7014; a Kenite or member of the tribe Kajin.
Kajin
Strongs 7014 Hebrew
Same as 7013 Kajin, the name of the first child. Cain, Kenite.
Kenite first appears in Genesis 15:19.
Then, having sent the crowds away, יהושע went into the house. And His taught ones came to Him, saying, “Explain to us the parable of the darnel of the field.” And He answering, said to them, “He who is sowing the good seed is the Son of Aḏam, and the field is the world. And the good seed, these are the sons of the reign, but the darnel are the sons of the wicked one, and the enemy who sowed them is the devil. And the harvest is the end of the age, and the reapers are the messengers. “As the darnel, then, is gathered and burned in the fire, so it shall be at the end of this age.
Matthew 13:36-40
Messiah links the devil and murderer in the same seed line.
They answered and said to Him, "Abraham is our father." Jesus said to them, "If you are Abraham's children, do the deeds of Abraham. But as it is, you are seeking to kill Me, a man who has told you the truth, which I heard from God; this Abraham did not do. You are doing the deeds of your father." They said to Him, "We were not born of fornication; we have one Father: God." Jesus said to them, "If God were your Father, you would love Me, for I proceeded forth and have come from God, for I have not even come on My own initiative, but He sent Me. Why do you not understand what I am saying? It is because you cannot hear My word. You are of your father the devil, and you want to do the desires of your father. He was a murderer from the beginning, and does not stand in the truth because there is no truth in him. Whenever he speaks a lie, he speaks from his own nature, for he is a liar and the father of lies. But because I speak the truth, you do not believe Me. Which one of you convicts Me of sin? If I speak truth, why do you not believe Me? He who is of God hears the words of God; for this reason you do not hear them, because you are not of God."
John 8:39-47
Cain was the first murderer in scripture.
There is no Cain shown in the genealogy of Adam.
This is the book of the genealogy of Aḏam. In the day that Elohim created man, He made him in the likeness of Elohim. Male and female He created them, and He blessed them, and called their name ‘Aḏam’ in the day they were created. And Aḏam lived one hundred and thirty years, and brought forth a son in his own likeness, after his image, and called his name Shĕth. And after he brought forth Shĕth, the days of Aḏam were eight hundred years. And he brought forth sons and daughters. So all the days that Aḏam lived were nine hundred and thirty years, and he died.
Genesis 5:1-5