Answer: The communities had very small populations.
In "Home Place Retold," we find out about the Mennonites, a group similar to the Amish, that came from Europe in the early days of the country. Among these families, it was common for siblings from one family to marry siblings from a neighbouring family. The reason was that, often, communities were very small and there was very little choice because of the small population. As the number of people grew, these practices became less common.
