Adam, Eve, and Incest
This is not a bad question. First, I'll answer the easier of the two questions: yes, you are correct that he Bible makes no mention of any other humans being created. So, the logical conclusion is that Cain must have married his sister.
Troubling? Why? In today's family structure where there may be 10 or so 1st cousins at a Christmas dinner, it's a bit disturbing to consider marrying any one of them. Not to mention the genetic disorders that result (I'll address that in a moment). But, try to imagine life outside the social stigma of incest we have today. Adam lived 930 years. Which, assuming he was virile from creation, means he could have fathered over 900 children in his monogamous relationship with Eve.
So, Cain didn't grow up with his sister. He's not in family photos with her. They didn't play together as children. In fact, she may be as strange to him as any other person in a crowd of 900. In that sense, it's not quite so... well... icky.
What about the Biblical ban on incest? Remember that law was not given for several thousand more years in Leviticus 18. It is not uncharacteristic of God at all to provide new or revised Laws to protect His people as time progresses. For whatever reason (and scholars can speculate that genetic devolution was the cause) God saw fit to forbid that which He once approved (Abraham married his half-sister).
I appreciate some other works on this topic found at:
http://godlorica.blogspot.com/2006/01/whom-did-cain-marry.html (particularly if you do the math as he suggests)
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