The Bible only tells us that Cain was the first man it does not state that he was the firts child to be born. He difinetely would have married his sister. As you can imagine they would be year apart and might not even have known each other. Also there was no law at the time and so it was not wrong to marry his sister.
There was a pre-adamite world! Man existed in that world as one of the beasts. I believe evolution happened in this world.
This world was destroyed and became void.
The account of the creation in the book of genesis is actually a recreation no wonder in the creation of man God made the statement "Replenish the Earth" The earth must have been there before and destroyed for God to use that term.
Hope thi helps