I watched this last weekend and was really disappointed.

SPOILERS

Basically, the truth is that the area was settled by eight families around a hundred years ago. Some of them had a predisposition genetically for twins. In that isolated location with such a small gene pool, that anomaly thrived through the several generations to today.

The documentary spent an inordinate amount of time on the whole mengele angle, which they knew to be false, and really only sprang this genetic thing at the last minute. Lousy way to do a documentary.