Diagnosis - inbreeding
Cure for those already born with heredity problems - none.
Cure for the future - abandon the community, leave, at once, and get out in the world and marry someone from outside the exclusive community.
It make take a few generations to reach the same problem level of “siblings & first cousins” but after awhile nearly all small exclusive communities develop inbreeding problems.
>> Cure for the future <<
A policy of deliberate racial miscegenation certainly would help. Maybe it would even eliminate the problem altogether in one generation.
Specifically, bring in a bunch of nubile Mormon youngsters from Africa and/or Asia and arrange marriages between them and the local kids. No local kid allowed to marry another local kid.
Still, something tells me this policy ain’t gonna win favor in rural Utah.
So back to the drawing boards?
Somewhere I read that when the railroads started up in the UK, the gene pool was rejuvenated as people were able to leave their homesteads and go far afield. Maybe an Urban Legend, but it sounds right to me. Any time the population of an area goes insular, things go downhill.