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.
You can always say to +Nom or jest/są +Ins.
can someone explain why this is "to" instead of "są" for "are"?
Just a choice of whoever wrote this sentence. It could be either "to rodzeństwo" or "są rodzeństwem".
In case you need more info, read here: https://www.duolingo.com/comment/16373167
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.
Actually, there is, I put everything I could find here: https://forum.duolingo.com/comment/16296174
On the PC Crown version, there is no "to" to select.
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.
how odd, "Brat i siostra to rodzeństwo." is exactly what i put and is marked wrong... ?