The Targaryens' propensity for arranging weddings inside their own House is further demonstrated by the Targaryen family tree being at the centre of House of the Dragon, which further begs the question of why they all wed one another.
. The Targaryens were known for marrying siblings, cousins, and other relatives together long before they arrived from Old Valyria, and this feature has been established since the events of Game of Thrones.
Aegon the Conquerer, the first Targaryen Westerosi monarch, wed both of his sisters and had a son with both of them. The other members of the Targaryen line were born through Aegon's son Aenys I.
King Viserys (Paddy Considine) marries his cousin Aemma Arryn in the movie House of the Dragon (Sian Brooke). The long-lasting and largely happy marriage of King Jaehaerys I (Michael Carter), the grandfather and forerunner of King Viserys, and his sister, Queen Alysanne.
There are major exceptions, like as the marriage of Lord Corlys (Steve Toussaint), a member of the House Velaryon, and King Viserys' cousin Rhaenys, who are, at best, very distantly connected due to their association with Aegon the Conqueror's mother Valaena Velaryon.
Another instance is the union of the parents of Queen Aemma, Rodrik Arryn and Daella Targaryen, who are not related by blood.
In any case, House Targaryen emphasises getting married within the clan. The Targaryens allegedly wed inside their family to maintain "the blood of the dragon" purity, according to the source novel for House of the Dragon, Fire & Blood, which describes the Targaryen dynasty between the reigns of Aegon the Conqueror and Aegon III.
The Targaryens have married each other numerous times throughout the history of the cosmos because House Targaryen believes that the purity of its lineage is a sign of superiority over other houses.
Before the Doom Of Valyria, the other highborn houses that were present on the Targaryen home continent held this view of inbreeding, and House Targaryen was no different.