but is clearly derived from the word for sister, not that for brother - correct, from a purely etymological standpoint, but "Geschwister" was used for siblings of mixed gender way before the whole gender question arose. Nothing against equality, but I find "Geschwisterkeit" a linguistic monster.
Edit - now I know why: Google yields zero hits on Geschwisterkeit. The correct word is Geschwisterlichkeit (10400 German-language hits).