How did Cain and Abel have children? Did they marry their sisters?
Indeed, Cain and Abel had to marry their sisters, considering that there were no other women around. King David writes, "The world was built with kindness." Our sages explain that this verse is referring to G‑d's kindness in allowing Adam and Eve's children to marry their own sisters in order to populate the species.
The Midrash tells us that Cain was born with a twin sister and Abel was born with two sisters. They each married the sister who was born with them. According to one opinion, it was actually a quarrel over who would get to marry the third sister that led to Cain slaying Abel.
The third generations of humans had no need for this loophole, because marriage between first cousins is not considered incest according to Torah law.
So why are these daughters not mentioned in the Torah? They are. Later in Genesis we are told, "And the days of Adam after he fathered Seth were eight hundred years, and he fathered sons and daughters"—though we are not informed of their identities. The Torah only records the names of those who were leaders of note, those who played a role in the biblical narrative, or the men who formed the chain of lineage connecting Adam to Noah (and later Noah to Abraham).