This entire discussion is being skewed by cultural bias, and tainted by individual point of view and perceived morality.
Personal feelings about incest, whether or not it is legal or illegal wherever you happen to be, and what is thought to be other's views on the subject will vary greatly depending on where you are and what your own life experience/education have been.
Did you know there are a number of entire cultures where incest is not a taboo?
There are cultures where people are introduced to sexual adulthood by their parents; a practice that has gone on for generations and is a fundamental part of their society. In some cultures it is other close or extended family, rather than parents, where older members teach the adolescents about sex and even give them feedback to help them improve for their future partner(s).
So how about this - If you personally have a problem with incest, or any other aspect of a particular game, novel, or other form of media, then make a conscious choice not to partake of those. If you have no problem with it, and believe that what two consenting adults do in their private lives, as long as it doesn't harm anyone else, is entirely their business, then enjoy the story and carry on.
Either way, don't presume that your opinion or point of view is the only one, or worse the "right" one.
Simply let others enjoy what they enjoy.