In "home place retold," why was it a common practice for siblings from one family to marry siblings from a neighboring family?

In "Home Place Retold," we find out about the Mennonites, a Christian separatist group that came from Europe in the early days of the development of America. 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, both due to their initial reduced numbers and their high mortality rate. This meant that there was very little choice of partner because of the small population. As the number of people grew, these practices became less common.

in this alternate universe, renaissance romeo is now seventeen, an apprentice to a glass maker in venice. modern romeo is planning to go to college. what major do you think he’ll elect?