"They are brothers."
I think it's context-sensitive; 兄弟です・きょうだいです by itself with no other context wouldn't particularly make sense… From what I've learned so far, it feels like です would be hard to translate literally with only 兄弟 to work off of. In an attempt to try though, if I'm not mistaken, I think it'd still be something like saying "brothers" but just in a confirmatory type of way? So I believe it'd sound a bit nonsensical if it's not linked to anything via extra words or context.
Times where I think it WOULD make sense is if someone already asked about something, like "what's your relation with them?" or maybe something like "are you cousins or brothers?" I think could both be responded to with 兄弟です。
Part of it could be that "siblings" isn't used too much in English, at least not casually. If they were both boys we would be most likely to use "brothers" in English. Even if they were mixed gender, many people would say "brother(s) and sister(s)" instead of "siblings".
Also, in Japanese there is the word 姉妹 which means "sisters", and I don't know if 兄弟 can be used for two female siblings, even if it could be used for mixed-gender siblings. I think maybe it can be used for "brothers" any time we would say that in English, or "siblings" in cases where we could also say "brother(s) and sister(s)", but it probably can't be used as a translation of "sisters". The English word "siblings" can be used to substitute all three groups in English, which might be why they want to emphasize the gender difference in the Japanese word, even if the entire difference isn't shown in this one sentence.
If you want to be clear that they are brothers you have to say 男の兄弟. There's also 女の兄弟 which is an another way to say sisters. Although compounds like 兄妹 and 姉弟 (either of which can also be read きょうだい BTW) get used much more, for some reason the former much more than the latter, oh and also 兄弟姉妹 for siblings which is probably what's getting shortened to 兄弟 as a sort of abbreviation? Oh 弟妹（ていまい）meaning younger siblings also exists and gets used a lot (possibly because there's a separate term for this in Korean?), but 兄姉 meaning older siblings gets used even less than 女の兄弟 (possibly because it's not polite enough to one's seniors?).
In this context it would be right, but かれたち is not common and I prefer you to use it かれら(they for mix genders) and かれたち( they for only boys) because if you use かれたち you would get confused very easily (they are used only girls so you could use かれら or かのじよたち). Hope it helps All the best :)
It would be かれらはおにいさんです (おねえさん is older sisters) and that's precisely why that would be incorrect. It would translate to "they are older brothers" which doesn't make sense. The word for brothers is きょうだい。(おにいさん is older brother) The word for sisters is しまい 。(おねえさん is older sister)
Therefore かれらはきょうだいです is the appropriate response for this exercise.
Usually, ～たち is softer (can be politer in most cases) and ～ら is rougher.
I recommend this article if you are really interested: