I’m sure there are siblings and family members who act on these impulses. I do not feel it’s common. The most common amongst this rare situation would probably fall under actions done while under some influence, with hormones through the roof.
What you’re likely unwittingly basing your belief on are sexual taboos. Incest isn’t a popular(arguable) category of porn because of the incest itself, it’s popular because of the taboo and excitement that comes with committing an act you shouldn’t. Someone who has these desires is not an inherently bad person. Even acting on a consensual interest isn’t a bad thing. None of this is evil or malicious. What it is to me is an act that has an underlying reason for existing. And I’d make a bet that many of the people who seriously consider incest as okay have:
Convinced themselves over time
Have trauma in their past that could be tied to that belief
It’s even possible that this is in some form a Paraphilic Disorder. Whatever the reason, I urge all individuals who are victims of their own minds to seek help if they can. Most of us never realize the weights we carry until we are given permission to take that weight off.