Yes, this is almost certainly illegal, depending on where this took place. Fortunately for weirdoes (like me) however, possession of the video is not illegal in basically any country where pornography is legal at all. Much in the same way that video footage of a burglary isn’t illegal in itself.

Like most things which are illegal, incest takes place all the time in the real world. If people weren’t doing it, there would not be a law written to forbid it. You can find news or police reports of examples of this.

There is also still a tiny chance that this is staged. It would have to be a very elaborate hoax though.

Also, I think it’s kinda hot.