The First Book of Adam and Eve
Chapter LXXVIII - Jealousy overcomes Cain. He makes trouble in the family. How the first murder was planned.
1 Then Adam said to Eve, "Behold the children are grown up; we must think of finding wives for them."
2 Then Eve answered, "How can we do it?"
3 Then Adam said to her, "We will join Abel's sister in marriage to Cain, and Cain's sister to Abel.