Rather than being afraid of his siblings, or relatives, why did Cain fear death by others? Who were the others?
Might Cain be what's now called Cro-Magnon? Might his wife be either Cro-Magnon, or part of the other... a Neanderthal?
If one of your brothers killed a brother you loved, you might be inclined to seek revenge. We don’t know how many children Adam had at this point.
One interesting interpretation of God’s statement seven-fold was not that God was protecting Cain, but a prophecy how Cain’s descendants would deal with those they have problems with. They would punish those they ruled over much greater than the offense given.
Watch this video, it might take a few of days to fit it all in. I found it interesting.
http://www.youtube.com/watch?v=spmOT2lYVVc