incest in game of thrones/hotd has always been associated with narcissim, with royal lines, their egoism and their obsession with blood purity. it's not just there for shock factor.
cersei sleeps with her twin brother bc she's a narcissist and bc she only cares about the advancement of her own house which is an attitude she inherited from her father. the lannisters are rotten to the core bc they can't love anyone but themselves and so they produce monsters (joffrey). there's a really poignant scene in the book where cersei sleeps with her maid (i think) and it's not bad but she cannot enjoy it bc "it's no good. it's never been good with anyone but with jaime" cersei can only love a mirror image of herself
the targaryens are the same, even the 'good' ones still believe they are closer to gods than to men, they think other people are beneath them and that they alone are born kings. they asociate themselves with the dragons bc they want to become dragons, in some cases they think they are dragons - when they are just humans like everybody else. that's why so many of their children are born malformed - they want to be dragons so desperately, they destroy themselves. the rot starts at the core. narcissim to the point of cannibalism, destruction of the self at the hands of the self
