Characters who are not normally siblings

Posted under General

https://x.com/okudo61_omori/status/2060368995367452968

This illustration contains four characters, in order:

As you can see, even though there are four characters, it's as if there were two of them, simply two copies of each.

This fan illustration was based on an illustration by the series' creator, released for Twins Day, in which the two copies of each character, instead of being simply versions from alternate universes, are siblings (i.e., there are two sets of twins).

Is there a tag describing this situation?

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