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)

twin towns, sister cities, twinned municipalities and other localities  : MORE DESCRIPTION HERE (Link to WP page)