Mid 19th century; earliest use found in Charlotte Yonge (1823–1901), novelist. From the name of Plymouth + sister, after Plymouth brother, Plymouth Brethren.
Definition of Plymouth sister in US English:
Plymouth sister
nounˌplɪməθ ˈsɪstə
A female member of the Plymouth Brethren.
Origin
Mid 19th century; earliest use found in Charlotte Yonge (1823–1901), novelist. From the name of Plymouth + sister, after Plymouth brother, Plymouth Brethren.