I think George R. R. Martin is just a sexual deviant and it shows through in anything he does. The incestuous relationship between Cersei and Jaime almost comes across as normal in the storyline.
We see the relationship through their eyes, and, to them, it is normal and acceptable. It also helped to destroy their family and their future.
GoT uses unreliable narrators routinely. That's why we hear from one group of people what a horrible, murderous rapist Rhaegar Targaryen is and what a wonderful guy he is from another. The information the reader gets changes depending on what information those narrators have/give.
It is normal, GRRM did not invent Incest.
Strip out all the nudity and sex and all you have left is violence and torture. That’s okay right? And GRRM must have secret desires to kill and torture people because he wrote about it once.
Royal families have always engaged incest to preserve the royal blood lines.
Martin didn't invent this tradition. He merely fictionalized it in his books.
What I find surprising is that you didn't know this fact, or bother yourself to look it up before posting.