I don't think it much matters why you commit incest in places where it's illegal. Porn (the recording and distribution of it) is just proof that you broke the law. I doubt there's a porn exemption in incest laws.
The point is there has never been an incest case related to porn that I've heard of. OTOH, there's been a few cases of underage girls in porn over the years, from Traci Lords to Bieyanka Moore, that have caused a stir.
Now, if you think the reason there hasn't been any controversy or legal cases related to incest in porn is because it doesn't occur, then you're naïve. It happens but no one, niether the law nor the companies, gives a shit.
Furthermore, there are very few if any people going to prison for incest if they're doing it with a relative that is close in age and ability; by which I mean incest not involving children, the elderly, or the disabled three groups that are considered vulnerable.