Aegon the Uncrowned and Rhaena (the Black Bride)

"Given the traditions and practices of House Targaryen, a match between his two eldest children must have seemed the obvious course to King Aenys. The affection between Rhaena and Aegon was well-known, and neither raised any objection to the marriage...
...The storm that greeted the king's announcement took them all by surprise, though the warning signs had been plain enough for those with the wit to read them. The Faitn had condoned, or at the very least ignored, the marriage of the Conqueror and his sisters, but it was not willing to do the same for their grandchildren."