First mention of the sibling incest motif:
She had always assumed that she would wed Viserys when she came of age. For centuries the Targaryens had married brother to sister, since Aegon the Conqueror had taken his sisters to bride. The line must be kept pure … Dragons did not mate with the beasts of the field, and Targaryens did not mingle their blood with that of lesser men.
Interestingly, this implies that Targaryen sibling marriage started in Westeros with Aegon the Conqueror (i.e. it was not a tradition followed in their previous home of Valyria).