When we read about Cain and Abel in Genesis, we don’t read about other children before them. However, after Cain murdered Abel and God cursed Cain, we read this verse.

And Cain had relations with his wife, and she conceived and gave birth to Enoch. Then Cain built a city and named it after his son Enoch.

Genesis 4:17 (NIV)

If we assume that Adam and Eve only had Cain and Abel to this point, we would be in error. We read about Cain’s birth because it was customary to document the eldest son’s birth. We read about Abel because of his significance as being pleasing to God and being the first person murdered.

Another reason why we know that Cain and Abel weren’t only Adam and Eve’s only children up to Abel’s murder is from this verse.

Unto the woman he said, I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee (emphasis added).

Genesis 3:16 (KJV)

When God cursed Eve, he also blessed her with many children. Hence, the multiplication of her “conception.” We read about Seth’s birth because he “replaced” Abel.

Therefore, the woman Cain was married to was his sister and not, as some might think, from another branch of people God created that we know nothing about.