The idea that the children of Adam and Eve were in incestuous marriages is, in my judgment, mere speculation, and one I do not find a reason to believe.
A clear example of incest in the Bible is Lot and his daughters who got him drunk and slept with him so they could have children, but I certainly don’t think the Bible recording this incident should be interpreted as an endorsement of such behavior. The Mosaic Law, which guided the morals of the people who wrote the Old Testament, clearly outlaws incestuous relationships, both with blood relatives and relatives by marriage.
Likewise, from commentary I have read the abomination among the Corinthians condemned by Paul as a misinterpretation of Christian liberty was very likely incestuous marriage.