It's pretty much illegal (yes, it's a law) around the world. Some countries, Germany, Sweden for example, there are certain restrictions on the law....but I doubt very much where you live, it's fine to fuck your mother, son or daughter and not be arrested and sent to prison if the authorities found out.
Nearly all countries do not consider cousins as incest (even first cousins)...some will allow same sex siblings etc....but don't be frightened and don't think it is not a criminal offence (hence illegal).
What payment processors are doing is ridiculous I agree, it's a depiction and where do you draw the line on that? Game of Thrones going to be banned or unavailable for purchase with your credit card etc? How about acts of murder in GTA? It's a very thin line....but when it comes to sexual acts, well, the religious loonies will always force their agenda.