First of all any, reference to Eve must depend upon Biblical exposition; that is, Adam and her were created on the 6th day. As the only two people in the then-world and with immediate responsibilities, we can safely presume God created them as adults, perfect adults. For instance, as a 25 yo adult she might have been only five minutes old.
If Eve was created from Adam's rib, that would mean that they were related and Cain and Abel were products of incest.
The concept of prohibited incest, does not come about until long after the flood. The logical reason for this is that Adam and Eve were perfect, having perfect genetic makeup, and therefore procreating would not be subject to the genetic defects that came later. As time progressed, those defects accumulated, so procreation of close relatives eventually became a biological problem.
