Translation:We are siblings.
"we are siblings" is the translation that would work for all the cases. "We are brothers" is also possible but this sentence does not imply that all siblings are male, only that one of them is, so could be incorrect in some contexts. "we are brother and sister" is also possible but in a specific situation: if there were only two siblings, being one male and one female (and those are not implied in the sentence). Therefore, "we are siblings" is the best translation to a sentence out of context like Duolingo's.
"Somos hermanos" could also be translated as "We are brothers" whiles "We are sisters" is rather "Somos hermanas"
It could be translated to "we are siblings" to account for multiple genders, but I'm not sure if Duolingo would recognize the word.
"Nosotros" is implied by the conjugated form of "ser", that is "somos". Hence, it is essentially redundant.
The verbs changes depending on the pronoun, then we don't need to say the pronoun
This can mean 'we are siblings' as well. This is what I answered and I got the question wrong, but I don't believe I should have.
Why is this "brothers" and not "we are siblings"? This seems inconsistent with the last question which used hermanos for siblings. Although it also took brothers, siblings seemed to be the preferred answer. (This is my very first posting.)