This is related to the old question (I guess it's supposed to be a stumper, which it isn't): Where did Cain get his wife?Not... quite. If there was only Adam and Eve, and then Seth? (Abel being dead, and Cain being sent away).
Then to get kids from Seth, it was Seth + Eve for awhile? And the, perhaps Adam and Seth's daughter(s)?
The story simply isn't all that clear-- but obviously for just two starting pairs, you are going to have to use multiple partners to get the population going.
Anything else simply does not work mathematically.
It's not that hard to answer....the answer is found @ Genesis 5:4.
Their DNA being near genetically perfect, such bonding between a brother and sister, or uncle and niece, or aunt and nephew, would not have deleterious results, as it does today.
Even Abraham, several generations later, married his half-sister Sarah and had healthy offspring.