https://bugzilla.wikimedia.org/show_bug.cgi?id=73534
--- Comment #7 from Fabian CommodoreFabianus@gmx.de --- Well we (or pywikibot for that matter) can't know if a site is a part of a family. How could we know that the site linked with 'de' on en.vikidia is in the same family as vikidia itself.
We could assume that ISO language codes are part of the family, as those are by default shown on the sidbar. But apart from that we don't: e.g. test.wikidata which is in the wikidata family.