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....
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)