defining relationships between terms in a taxonomy
The relationship functionality exists in Drupal core - no need for a contributed module.

Go to /admin/structure/taxonomy/YOUR_VOCABULARY/fields and add a new field of type Term reference, click Save.

On the next screen 


change Number of values from 1 to Unlimited
choose the same vocabulary you added the field to in the Vocabulary dropdown.


Then edit each term and add the neighbouring regions to it.