"A brother and a sister are siblings."
Translation:Brat i siostra to rodzeństwo.
If there is a plural verb, why wouldn't it be rodzeństwami? Is rodzeństwo similar in number to rodzina?
yes exactly. a whole set of "siblings" is "rodzeństwo', more groups of siblings are "rodzeństwa".
Why do both of these work? I would have thought it would be the instrumental only.
This link is really useful and interesting -- but how can I find it when I move on the next challenge. Is there a "master book" of this kind of useful information on Polish; and if so, when can I find it easily? Grazie mille.
The starred (and actually the only two accepted) options are "Brat i siostra są rodzeństwem." and "Brat i siostra to rodzeństwo.", if you are able to create one of those, everything is fine.