Not quite clear on the concept

We just got back from a week in the Dordogne, a part of France we didn't know at all before this trip. It's part of Aquitaine, which was English territory until the Hundred Years War. We noticed that almost every village had a sister city…in France. That's not normally the way it works, but there may be a reason. Most of the sister cities were villages in Alsace and Lorraine. A lot of people either fled from or were forced out of those two provinces at the beginning of WW II and wound up in the Dordogne, so I guess finding a sister city over yonder was a way of keeping folks close to their roots. Besides, the rest of France considers them kind of foreign anyways.