Its science
Once you start inbreeding within the family
You start getting all these genetic problems
Just take a look at the monarch era
Tons of inbreeding
Inbreeding results in homozygosity, which can increase the chances of offspring being affected by deleterious or recessive traits.[3] This usually leads to at least temporarily decreased biological fitness of a population[4][5] (called inbreeding depression), which is its ability to survive and reproduce. An individual who inherits such deleterious traits is colloquially referred to as inbred. The avoidance of expression of such deleterious recessive alleles caused by inbreeding, via inbreeding avoidance mechanisms, is the main selective reason for outcrossing