I believe I've covered the whole situation on incest but here goes again.....
The children of the children of Adam and Eve wouldn't be mutated and deformed because the evil and bad things hadn't taken hold yet. Adam and Eve's children probably would have mated at say 16 or 18 years after the Fall. The first death was at the Fall so that was the second bad thing that had happened. Things such as STD's and deformities hadn't had time to come into being because sin hadn't been around for long. After those children had children, they wouldn't have to marry their siblings, but maybe like their cousins. There is less of a chance with cousins because they aren't as genetically close as siblings. After the next generation, they could mate with aunts, uncles, second cousins, or others. There is even less of a chance, and by then, there is a lot of people since they all had long lives ( Adam lived for over 500 years). Proving even that sickness hadn't taken effect.