Relationship between content types
You can use Corresponding Entity References to achieve this.

Follow these steps:


Create two content types: Region and City.
Insert entity reference field: Cities in Region and Region in City.
Now enable Corresponding Entity References and update its configuration by going on your site: Configuration >> System >> Corresponding entity references.
Check this configuration: Correspond Region on node(s) of type city with Cities on node(s) of type region.

Now in next tab Updating existing entities of entity type: node.