Jewish tradition says that Cain and Abel both married their sisters. It says in some sources th at these sisters were born at the same time as each of them, and also that Abel had two sisters and Cain had a single one.

The prohibition on incest (for all of humanity, not just the Jews) was not in place at the time, for the reason that the earth needed to be populated.

And no, I'm not Jewish, I'm converting.