Secondary Edit - Restoring question as the Mods have requested it.

Could you clarify the question? By Sister cities do you mean

A - the twinning of geographically separate cities? (like in the UK you see signs on city limits saying "Welcome to $town, twinned with $otherTownInOtherCountry")

B - The phenomenon of two geographically close cities becoming linked with economically dipsarate conditions, such as Gloucester and Cheltenham in the UK or Marseille and Aix-en-Provence in France