"Are you brothers?"
It's very easy to understand the difference if you know the kanji forms of those two words. Here are the four kanji you need to know:
- 姉 = "older sister"（あね）
- 妹 = "younger sister"（いもうと）
- 兄 = "older brother"（あに）
- 弟 = "younger brother"（おとうと）
The hiragana shown in the list above are the KUN readings of each of those kanji. KUN readings are the native Japanese words which are written with that kanji.
ON readings are just the sounds of the kanji. Rather than being words, ON readings are essentially meaningless on their own. For words formed from a combination of multiple kanji, ON readings of the kanji are what were used for coming up with the pronunciation of these kanji compound words. Anyway, here are the sounds (ON readings) of those four kanji:
- 姉 (older sister) = し
- 妹 (younger sister) = まい
- 兄 (older brother) = けい or きょう
- 弟 (younger brother) = てい or だい
Now all of that's out of the way... Here's how those two words you're asking about are written in kanji:
姉妹 (しまい) = "sisters"
lit. older sister (姉, し) + younger sister (妹, まい)
兄弟 (きょうだい or けいてい) = "brothers"
lit. older brother (兄, きょう) + younger brother (弟, だい)
- BONUS -
- 兄姉 (けいし) "brother and sister"
- 兄妹 (けいまい) "older brother and younger sister"
- 姉弟 (してい) "older sister and younger brother"
- 兄弟姉妹 (きょうだいしまい) "siblings"
Couldn't this (きょうだいですか) be interpreted as "Are you a brother?"?
To be more specific I wrote 兄弟たちですか but DL didn't accept that. Is it wrong to append たち to the noun like that?