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").
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).