Well in a society such as the one in which we live, where post-modernism is the predominant philosophy and morality is an entirely subjective concept, there is no reason for anything to be immoral, unless perhaps it harms someone else. I think that's crap, but moral subjectivity is a very popular way of looking at the world right now, and you can't just take the parts you like.
On the other hand, if you have a belief system that provides a basis for an objective, absolute system of morality (read: God), then you have a system laid out for you. And I think in most of those cases, incest is wrong on a basic level no matter what the circumstance.