Well, it hasn't worked out very well for European royalty

Last of the habsburg line

The Economist: Cousin marriage is probably fine in most cases

The title for the kings of inbreeding probably belongs to the Habsburgs, who took generations to die out, but die they did, after ruining their countries. here’s how they ended:

His parents were Philip IV of Spain and Mariana of Austria, who were born to first cousins and were uncle and niece. 

They were part of the House of Habsburg, which produced generations of Austrian and Spanish kings and queens, and occupied the throne of the Holy Roman Empire from 1438–1740.

However they were also masters of inbreeding and frequently married close relatives such as nieces or first relatives. This in turn led to ill health and a high rate of infant and child mortality.

Centuries of inbreeding within the House of Habsburg culminated in Charles II.

According to Ottinger and Voigtländer's calculations, he was 'more inbred than the offspring of siblings would be'.

Historians believe inbreeding contributed to Charles II's host of medical issues, as well as his inability to produce an heir. Despite having married twice, Charles II died childless, which sparked the War of the Spanish Succession between 1701 and 1714.

Charles's physical disabilities are well documented. He only started talking at the age of four, the same year he became king, and walking at the age of eight. Woods described Charles II as an 'imbecile'.

His tongue was so big he had difficulty speaking and drooled, and when he died in 1700 aged 38, the coroner found lungs corroded and his intestines rotten.

Analysis shows that Spain suffered under Carlos II's rule. Woods noted he reigned over a country characterised by 'misery, poverty, hunger, disorders, decline, especially in agriculture, finances and strength of the army'. 

Do we need to mention that nitwit Harry, of the House of Saxe-Coburg and Gotha?