Elohim had blessed the human beings He had created and allowed them to have children. That way A'dham became the father "of a son" in his own likeness, "after" his own image. We were not told this about Cain and Abel simply because the text did not wish to discuss them at length. In this chapter 5 Moses goes straight to the third son of Adam, Enosh. It is noted that from that time the people started to call upon the Name of Jehovah God.
What the Scriptures tells us about how God reacted on Kayn (Cain) his act of offering and killing his brother
We come now to the position that the 1st man had relations with his wife Eve, and she conceived and gave birth to Cain, who when older brought offerings to please God.