Aegon the Conqueror weds his sisters

"Aegon had two trueborn siblings; an elder sister, Visenya, and a younger sister, Rhaenys. It had long been the custom amongst the dragonlords of Valyria to wed brother to sister, to keep the bloodlines pure, but Aegon took both his sisters to bride. By tradition, he was expected to wed only his older sister, Visenya; the inclusion of Rhaenys as a second wife was unusual, though not without precedent. It was said by some that Aegon wed Visenya out of duty and Rhaenys out of desire"
- The World of Ice and Fire, George R.R. Martin

The wedding gowns were based upon those worn by Daemon and Rhaenyra on their valyrian wedding on House of the Dragon