According to Humon, this concept came to be because the stereotypical Swedish woman is very different from the stereotypical Swedish man. Sister Sweden is thus by far the most prominent sister, and the only one to have her own tag "Sister Sweden" (Sister America is tagged as "Sister America" in only one of her appearances). Interestingly, two strips before "Meet my Sister" was "Eurovision Winners", in which Sweden and Iceland had to dress in drag (representing Eurovision contestants Charlotte Perrelli and Yohanna respectively) as Sister Sweden and Sister Iceland has not been introduced yet, which may have contributed to the idea of having sisters. On the other hand, some strips after "Meet My Sister" still have the male characters dressed in drag, such as "Frozen Scandinavia", in which Norway and Svalbard dress up as Anna and Elsa.
Most of the Sisters look and act simply like gender-flipped versions of their brothers. The major exceptions are Sister Sweden and Sister America. Since only a dozen or so sibling counterparts have been shown in the comics, fans love to speculate about the others and theorize about how they might differ from the character we know. Not much is known about the siblings of any of the characters aside from America, Japan, and the Nordic countries. The other sisters to be shown so far have made only one or two appearances apiece and have had little or nothing in the way of dialogue.
The countries for which both siblings have made an appearance include: America, Australia, Brazil, Canada (portrait only), China, Denmark, England, Faroe Islands, Finland, France, Germany, Greenland, Iceland, Japan, Netherlands, Norway, Russia, and Sweden.