He was a brother to Abel and Seth. There is no question of that - but he was not the son of Adam - Seth and Abel were.
PP, God says that Cain is Abel’s brother. The scripture clearly says that Adam was Cain’s father. You must decide that the ABSOLUTE CLARITY of the story narrative of Cain’s conception overrides any other obscure theory. One principle of bible interpretation is that “the clear interprets the obscure.”
Ge 4:1 - Adam lay with his wife Eve, and she became pregnant and gave birth to Cain. She said, “With the help of the Lord I have brought forth a man.”
Ge 4:2 - Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil.