After releasing No Closer to Heaven in 2015, The Wonder Years spent two years on a global tour to promote the album. Throughout the tour lead vocalist Dan Campbell was creating what would soon be the band's next work of art. According to Remfry Dedman of The Independent, Sister Cities emerged from this process as an album focused on unity in a fragmented world.

The theme of unity is central to Sister Cities. During their travels, Campbell noticed similarities between his life and those of people from different backgrounds. The album's title originated from a memorable experience in Santiago, Chile, where a cancelled tour stop led the band to play an impromptu show for about 100 people. While waiting for a bus at a nearby station the band was picked up by fans who asked them to play this other show.

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