Yup, just as I feared...its almost a useless property as it stands now. No clear definition. Confusion is rampant among everyone. And where I think things could be vastly improved...is here....
Wikipedia has some great pages that typically describe the Property. https://en.wikipedia.org/wiki/Twin_towns_and_sister_cities is one of those....yet, it takes 4 clicks to get to it from the Sister City property https://www.wikidata.org/wiki/Property:P190 and then you finally have a better description or intent of what the authors of the Sister City property were trying to describe.
Improvement idea and Suggestion to Wikidata admins: Let Descriptions allow (and show) the link directly to a Wikipedia page describing more fully the idea of the property.
sister city (P190) https://www.wikidata.org/wiki/Property:P190#mw-head https://www.wikidata.org/wiki/Property:P190#p-search twin towns, sister cities, twinned municipalities and other localities : MORE DESCRIPTION HERE (Link to WP page)
Thad +ThadGuidry https://www.google.com/+ThadGuidry