Will Esbern

“The tradition amongst the Targaryens had always been to marry kin to kin. Wedding brother to sister was thought to be ideal. Failing that, a girl might wed an uncle, a cousin, or a nephew, a boy a cousin, aunt, or niece. This practice went back to Old Valyria, where it was common amongst many of the ancient families, particularly those who bred and rode dragons. The blood of the dragon must remain pure, the wisdom went. Some of the sorcerer princes also took more than one wife when it pleased them, though this was less common than incestuous marriage. In Valyria before the Doom, wise men wrote, a thousand gods were honored, but none were feared, so few dared to speak against these customs.”
― George R.R. Martin, Fire & Blood