Complex sentence: There are several Brethren groups that are not related to the Schwarzenau Brethren movement, such as the Plymouth Brethren that arose in England and Ireland early in the 19th century, and the United Brethren originating in 18th century Pennsylvania with William Otterbein and Martin Boehm.
Split into simpler sentences: There are several religious groups named Brethren that are not related to the Schwarzenau Brethren movement. The United Brethren originated in 18th century Pennsylvania with William Otterbein and Martin Boehm.