Sister Cities International is the organization that handles sister city relationships.
There isn’t really a set way it happens. Sometimes it’s because of existing relationships between city leaders. It can also be started by private individuals or groups or have connections to the other city. SSI also helps cities find partners.
Then basically the mayors or city leader signs and agreement creating the relationship. There isn’t necessarily a set agreement. Cities are more or less free to define their partnership as they want and it’s not exclusive.
You can learn more [here](https://sistercities.org/about-us/what-is-a-sister-city-3/).
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