Display nodes with the same parent node
If it was me, I'd probably use a Region reference on the City so that you only need a single-value field, but this should be quickly accomplished in Views either way.

Assuming a City reference on the Region:


Create a View of Content entities of type City with a Block Display.
Add a Node ID Contextual Filter with Default Actions set to Provide Default Value > Content ID from URL.
Add a required Relationship to Region/Content using your reference field.
Add your reference Field to the display using the Relationship and uncheck/disable Multiple Field Settings > Display all values in the same row.
Add a Contextual Filter for your reference field using the Relationship with Default Actions set to Provide Default Value > Content ID from URL. Then check/enable the More > Exclude option.


Now just place the block in a theme region on your site so that it shows up on the City page. #1-4 should get a View showing all Cities that share a Region with the current one. #5 should remove the current one from the results.