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We can only speculate about what happened, at the beginning. Be careful: Don`t deceive yourself, with your own theories.
I think that Seth's wife might not be his sister since it seems to have other people who were existing at that time rather than Adam's family Ge:4:15.These are people whom may harm Cain if Cain would not have God's sign of continuing to live.
Agreed. But God also gave us deductive reasoning -- common sense.