definition: see: wikipedia:Town twinning
The network making up 'sister cities' should be mapped. Are there clusters of cities, mimicking human level social groups? Or is it more evenly spread?
Note that mapping this is difficult without datamining wikipedia at the moment. There is not yet a single resource with all twinned towns.
I have a small beginnings of a google map for this, but really it requires an automated approach. (making a google earth KML file? 
The curiosity for this is not only to look for clusters (or not) within the "family tree", but to then be able to identify "cousin" cities (as I call them. first cousin cities share a city, but no direct link. This usage is not analagous to human family trees, but is chosen as 'cousins' are on the same generational level as oneself). For example, Canberra and London are cousins, by virtue of both being sister cities to Beijing...