This is an interesting question. The thing to always remember when both asking or answering such a question is.. "how does this topic fit into my world view?"
There are many answers that you may get to this and many will be "sophisty"...[clever augments to support something that you have already decided to believe to the exclusion of all other possibilities.]
My answer reflects my world view, but at least I can assure you that during my 4 years at Bible college, I did examine all perspectives and did have an open mind before doing so.
In short, the answer depends apon whether you take a view that all the content of the Bible is to be viewed as "Historical". That is to say, there is no room to accept that God may have revealed himself or aspects of His nature by allegory or metaphor or poetry etc.
If this view is accepted than mankind needs to explain questions such as yours some other way in order to keep a consistency within what we know God has said about what is right or wrong. One such explanation may be that "incest was not actually forbidden" at the time.
The other way to approach it would be to adopt a world view or paradigm that accepts the notion that God has used many different methods of communication within the Bible, such as historical, instructive, predictive or prophetic, allegory, poetry etc, as primary modes within each book or passage, and that despite the mode used God is always consistent in revealing something about Himself and what He is communicating to us.
If this view is adopted, it becomes less important to focus of the mechanics of making sense of the incest question, and more important to seek out what it is that God may be saying about Himself and how He wants people to interact with Him.
My personal view is in the second camp, as it is my understanding that the Bible has never been a collection of books in which Gods primary desire was to reveal the nature of man [a situation in which man must constantly justify why humanity's actions such as incest exist despite knowing it is wrong]... but rather, it is God revealing to mankind about Himself and counterpointing it with examples of the nature and danger of sin.
Put simply, when I read passages about Adam and Eve, I believe that even if I came up with some legalistic or mathematical explanation or loophole to explain curly questions, it never leaves me feeling enlightened, or closer in understanding God's nature and how to apply it to my life.
By taking the other view, it allows me to not get bogged down in feeling so "clever" with my technical explanation, but rather allows me to seek areas in my life that need improving in contest with the overall theme of the chapter.
I hope this has been of some help to you.
The Bible was not designed to be a "rubics cube" only to be solved at all points by a minority of super smart theology majors.
The Bible was meant to have the same meaning to all people at all points in history..no tricks...no secrets only known by ancient peoples long gone...no one needed to sort and sift and simplify for the masses...just helpers and teachers to guide others slowly in their search for the meaning of their existence.