If you are talking about Adam and Eve.. yes, they did sleep with each other.. and their children slept with each other...
On the other hand... If it was you and your sister alone in the world.. with nobody else around.. and you should either pro-create or die off.. would you do it out of a need? Or just let the human race die off?
Let us be a little bit more crazy
Suppose the earth were to be destroyed, and you were able to get away on a spaceship with your family and land on a planet full of water and oxygen...
Your parents can not procreate anymore....
Would you start a new generation with your sister? OR simply go the way of the Dodo?
There is a difference between a "need' and just "lust".
Besides, once Adam and Eve "sinned" in the Garden, they were "living in sin" ever since. They were NOT married for starters. And their children inherited their sins..and their Children's Children.
Once this whole "chain of events" got started, it is not too far fetched to imagine that they would do such things as incest and brothers killing each other (Cain and Abel).
The end.
