Satan was the father of Caine

 Pirkei de Rabbi Eliezar (21:01). Sammael riding on the serpent came to her, and she conceived; afterwards Adam came to her, and she conceived Abel, as it is said, "And Adam knew Eve his wife". What is the meaning of "knew"? He knew that she had conceived. And she saw his likeness that it was not of the earthly beings, but of the heavenly beings, and she prophesied  and said: "I have gotten a man with the Lord".