i love how everyone is turning their nose at the incest of the Lannisters.
do you not remember Viserys said to Daenerys? ''if only you were born older'', why? so she could have married their older brother Rhaegar Targeryan,
who was instead married off to Oberyn Martell's sister, (a feat which many believe is one of the reasons Tywin sided with the rebellion of Rob Baratheon)
Incest is very common in Game of Thrones, the Targeryans regularly practiced this to keep power in the family, basically similar to almost every monarchy in most civilized countries if you date their histories back
Cersei and Jamie, just seem to be doing it behind closed doors, whereas the Targeryans publicly married siblings off