. . . and their two sons (no daughters) of course!
According to the Bible, they had several sons and daughters but only three are named.
Incest was not defined until much later. When it was defined it went much further than the civil laws we have nowadays and didn't just concern blood relatives, for example not getting jiggy with stepchildren or late siblings' spouses. Pity it isn't still like that now imo, anything else confuses family relationships. It is still legal - though discouraged - for first cousins to marry. The Royal Family stopped doing that after Queen Victoria's time and the Catholic Church forbids marriage between first cousins. Distant cousins is OK.
Incest is horrible but I can quite understand that, in the long ago days of the Bible, when there were undoubtedly less genetic problems, close relatives married and had children. If they hadn't, there would have been very few people around. It wasn't incest then, there was no such word or concept. We cannot judge early man by the standards of today.