Shouldn't "They are a brother and a sister." be accepted too since Russian has no articles?
I don't know about that..."they are a brother and a sister" sounds pretty unnatural in English, imo. "They are brother and sister" is what I've always heard.
Unfortunately, we have to accept a lot of questionable wording (like "In the box there lies a lamp" instead of "There is a lamp in the box").
But the other should be the one to but it would say how it should be.why wouldnt it be acceptable?
"They are brother and sister" means they are eachothers brother and sister. "They are a brother and sister" does not directly indicate if they are each other's brother and sister or someone elses. Even if there is no articles in Russian the sentence is incorrect in English and probably is why it's marked wrong.
How to refer to siblings, without telling the genders? In romance languages, we use just the masculine form in the plural (like "hermanos" in Spanish, or "irmãos" in Portuguese, or even "fratelli" in Italian). How to do that in Russian?
There is no common way to tell people are siblings without using the words "brother" and "sister". You can, of course, say that people "have the same parents" but that would sound needlessly vague
A dash is conventionally used when both the subject and the predicate have nouns BUT not when one of them is a proonouns—or both are pronouns (though the author of a sentence can still use one if they want to make the sentence more dramatic).