Abandoning sensual misconduct, he abstains from sensual misconduct. He does not get sexually involved with those who are protected by their mothers, their fathers, their brothers, their sisters, their relatives, or their Dhamma; those with husbands, those who entail punishments, or even those crowned with flowers by another man.
http://en.m.wikipedia.org/wiki/Five_precepts
Based on this and the general understanding that a wrong action is determined by its intention, wouldn't incest between two consenting adults be seen as moral (although not conductive to nibbana, like all sexual activity) in Buddhism?