Why did Valyrians do incest?
Why did Valyrians do incest? So, we know that the Targaryens, the last of the Valyrian dragonlords, practiced incest to keep their bloodline pure.
Why did the Targaryens marry each other? Aegon Targaryen united the Seven Kingdoms of Westeros with the help of his two sister-wives, Rhaenys and Visenya. The Targaryens often intermarried to keep the bloodline pure, but that tradition could result in nasty consequences.
Why are Jaime and Cersei together? Jaime and Cersei bought into this completely. They view Lannister as the golden standard and everyone else as second-class and a potential enemy. So them choosing the lover from their own family makes total sense. Especially given the fact that Tywin did it too.
Did Cersei sleep with her son? When Cersei has an affair with Lancel Lannister, their cousin. Cersei sleeps with their cousin Lancel throughout seasons one and two. Lancel does everything she asks him to, including causing the death of King Robert. In season five, Lancel confesses the affair and the death of Robert to the High Sparrow.