It has always been my understanding that Adam is representative of the first men, collectively, and Eve is representative of the first women, collectively. The specific points of their familial stories, especially concerning the slaying of Abel and the geneologies, are reference to the specific men and women from whom the Jewish race, God’s chosen people, were descended. Therefore, there is no reason to wrestle over any questions of incest because, more than likely, the first generation of humankind was not having incest.
- God created man and woman, therefore, he knows how genetics work.
- Incest has a much higher chance of genetic mutations, sterilization, etc.
- Continued incest can result in the complete genetic baselining of a community making further procreation impossible.
- Knowing this, we can safely believe that God would not create man in a situation that would require all future generations to be descended from a single genetic pairing.
- Therefore, God being God would not have created a single man and a single woman to “be fruitful and multiply.”
- Finally, we can deduce that God created many men and women and chose a specific lineage from which to derive His chosen people.
As for who Cain married, there is speculation that Adam’s legendary first wife Lilith was also later the wife of Cain. I do not believe this, but it’s there for your consideration. But, apart from a handful of women, women are almost entirely ignored if not completely written out of the Bible, so to answer the OP’s question, it is best not to go searching for an answer but to answer honestly–we don’t know to whom Cain was married, but it probably was not his sister, or his niece, or anyone else that would violate the reason and sanity of God (cf my logical argument above).