Cain married one of his sisters (which would also be Abel's sister) and Abel married one of his sisters (which would be Cain's sis too). It sounds really sick and wrong, but that is only because humanity has run its course and inbreeding (as we call it) produces malformations in the dna of children, but those country bumpkins deserve it, for they didn't have to marry and have babies with family. Getting back to Cain and Abel's wives, their dna was near perfect and having kids with their sisters was perfectly alright, and yes seth did that too as well as other sons until the family started to branch off into sections and went off into their own ways and then men and women of no relation began to have kids together. Here's a history lesson: Cain and Abel's dad was Adam and he was the first man, and he and his wife (their mother) was Eve, and Adam and Eve sinned against God, and God told them that all that eat of the forbidden fruit would die (as Adam and Eve both ate that unknown fruit after being deceived by the serpent/Satan), and from Adam and Eve on to us, we all are slowly dying, and this gradual death causes our DNA to corrode from within, and this sin nature degeneration began with Adam and Eve and continued through Cain, Abel, Seth and humanity until the current generations of our day. This sin nature/ dna corrosion also affects our bodies in other ways, such as in our bone density and our ability to sustain damage, as our bodies have been crippled by this sin nature, which is why only the healthy ones get to live past 70, as many die in birth, or well before they get to live "normal" lives, due to car crashes, crime (sin nature causes the degenearation of the human mind), diseases and all things harmful. However, all Christians will die and go to heaven and get new perfect bodies! Yes, I am saved, and that is my future!