The answer to this is easy, though dealing with it as a listener is not quite so easy.
Adam and Eve's kids married each other. They had to because there was nobody else to marry! (That's the easy part)
The natural question that then arises is, "But isn't that wrong?"
So the hard part is accepting that there was no rule (at this stage of history) that said it was wrong to marry a sibling. Some speculate that since it was much closer to the fall, the genetics weren't quite so muddled with mutation. This meant that harmful genetic mutation wouldn't compound the way it does with inbreeding today. Either way, there was no rule until much later.
What is more important in this objection isn't the question and answer, it is the ability to accept an answer that is based in a very different morality than that which we are used to today.
Some more reading from www.creation.com: