Everybody has a genetic weakness of some kind only they are not expressed.
Most genetic weakness goes unexpressed because a person has only received that bad gene from one parent.
When a person receives the bad gene from both parents the genetic disease is expressed.
Diseases such as hemophilia (Europe’s Royalty) and sickle cell anemia (African Americans) are such genetically transmitted disease.
These diseases become prominent when isolated populations of small numbers interbreed.
Europe’s royalty and African Americans haven’t died out because there numbers are in the thousands and millions respectively, but this tribe’s numbers are far too small to be sustained with out an influx of outside genes. A number I seem to recall reading is about 50,000 for a minimum sustainable breeding population.
...Or do not matter in the circumstances. Really harsh conditions have a way of filtering out genes that tend to leave one of ill health or debilitated. Having large families also leaves a lot of room for continued family lines with recessive issues. That said, the effects of inbreeding genetically, are quite overstated in the common understanding. Most of what genetic diversity gives a population is the ability to adapt to varying conditions.