Incest porn is not about incest, it is about close platonic long term bonds. It is essentially "platonic best friends all of a sudden becoming sexual" porn with a clickbait title.

The incest dynamic is really effective because it is one of the few plot hooks which can overcome porn logic. In most any porn you see, if there is a guy and a girl in the same room, they are going to have sex. There is never a situation where they aren't going to have sex. Furthermore, there is never a situation in a porn where two individuals are not going to want to have sex with each other....

Except what if that other person in the room was your sister?

For whatever reason, that is an exception to porn logic. It creates a situation in which there can be two individuals in a room who don't exactly want to have sex with each other which is believable. If it is a brother/sister sort of thing and the sister is trying to seduce the brother, it is believable that the brother might try resisting the sister. It can create a lot of sexual tension because there is a believe resistance. If the sister accidentally walks in on her brother and sees him in the shower, and she doesn't quite look but then can't help but taking some peeks, well that is believable because the taboo aspect of it makes sense. In almost any other non-incest porno the only possible way to handle that would be the girl seeing the guy and then walking in and giving him a blowjob.

What is interesting about this is that there is almost never a blood line, it is always step-sister, and that is because being actually related isn't what is important to this. It isn't about that person being a biological sister, but more about them being "like" a sister.

Why is that effective? Well, it opens up the avenue for people to project their own real life situations onto the porn. In everyday life, especially men, have friends who we are attracted to and want to have sex with, but the relationship is really only platonic. I've had attractive female friends who I have been very close with, so much so that they were "like" my sister. The way incest porn plays out, it is very similar to how the sex would play out if one of my attractive female friends who was like my sister started to come on to me.

There are many different avenues that these porns can go and they kind of borrow from different relationship dynamics. Father daughter can be for girls who want to be with an older man, maybe there is an older man they want to experience but feel like it would be too wrong because they are "like" a father to them, but it can also be effective for older men who know young attractive women who they are "like" a father to. You can go into all the different dynamics.

When you really start to see it from this perspective, it is put in a similar category to where two female best friends end up having sex. It is believable in porn logic for there to be two girls in a room and for one of them to not exactly want to have sex with another girl and to show some resistance, or for both of them to show some resistance and for them to just fall into having sex. This can work for a lot of girls viewers because they may have sexual feelings towards some of their female friends, and recreates some of the dynamics. It can work for male viewers because they can imagine that dynamic being at play with some of the people in their life.

There are other porn categories which play off this as well, like teacher/student porn. It creates some sort of strong enough reason as to why a man would resist sex, but also for anyone to resist sex. It plays on certain real life roles and boundaries which people find relate-able.

There is so much more to this answer and avenues I could go down, but that is the rough idea of it.