The Talmud in Sanhedrin 58b says that Cain married his sister.
For those who are wondering - who said that Cain had a sister; she doesn't
seem to appear anywhere in the Biblical text, please refer to Rashi's
comments to Genesis 4:1 where he quotes the Medrash (which also appears in
the Talmud Yevamot 62a) that Cain and Abel each were born with a sister (in
fact the Medrash says that Abel had 2 sisters!)
Uri Portal