http://www.answersingenesis.org/There is even an "answer" to your question
"...Thus, there was only one man at the beginning—made from the dust of the Earth (Genesis 2:7).
This also means that Cain’s wife was a descendant of Adam. She could not have come from another ‘race’ of people and must be one of Adam’s descendants....
Even though only these three males are mentioned by name, Adam and Eve had other children. In Genesis 5:4 a statement sums up the life of Adam and Eve—‘And the days of Adam after he had fathered Seth were eight hundred years. And he fathered sons and daughters.’ This does not say when they were born. Many could have been born in the 130 years (Genesis 5:3) before Seth was born.
During their lives, Adam and Eve had a number of male and female children. The Jewish historian Josephus wrote that, ‘The number of Adam’s children, as says the old tradition, was thirty-three sons and twenty-three daughters.’
The Bible does not tell us how many children were born to Adam and Eve. However, considering their long life spans (Adam lived for 930 years—Genesis 5:5), it would seem reasonable to suggest there were many! Remember, they were commanded to ‘Be fruitful, and multiply’ (Genesis 1:28)."
http://www.answersingenesis.org/home/area/tools/cains_wife.aspThere is quite a long explanation - but this is the main idea. It was ok to marry siblings, then (since humanity was just starting out) - and there were lots of siblings to marry.