There are several ways to let people know whom the siblings are when talking about them. One way is to use the counter 番目 (banme) to indicate where in the order the siblings are.
４人兄弟の３番目の女の子 (yonin kyōdai no sanbanme no onna no ko) = the third girl of four siblings
２番目の妹 (nibanme no imōto) = the second little sister
Another way is to use words that denote the order of birth.
長男 (chōnan) = firstborn son
次男／二男 (jinan) = secondborn son
三男 (sannan) = thirdborn son
長女 (chōjo) = firstborn daughter
次女／二女 (jijo) = secondborn daughter
三女 (sanjo) = thirdborn daughter
If the speaker has two younger sisters and wants to say "One sister is a... and the other is a ..." then it is possible to say １番目の妹は…です。そして２番目の妹は…です。(ichibanme no imōto wa ... desu. soshite nibanme no imōto ha ... desu) = "the first little sister is ... and the second little sister is...".
If it is known that the speaker has only two younger sisters, then he can say 長女は…です。そして次女は…です。(chōjo wa ... desu. soshite jijo wa ... desu) = "the firstborn daughter (of my parents) is... and the secondborn daughter (of my parents) is..." if the speaker is male or 次女は…です。そして三女は…です。(jijo wa ... desu. soshite sanjo wa ... desu) = "the secondborn daughter (of my parents) is ... and the thirdborn daughter (of my parents) is..." if the speaker is female.
兄弟 (kyōdai) can mean both brothers as opposed to 姉妹 (shimai) and siblings in general. When talking about siblings, it is possible to write 姉妹 / 兄妹 / 姉弟 and read it as kyōdai. Please note that the person who is speaking counts as one of the siblings. "I have three siblings" --> "we are four siblings".
A way to explain that the speaker has no brothers is 男兄弟はいません (otoko kyōdai wa imasen) = (I have) no male siblings. If the speaker is female then using 姉妹 (shimai) = "sisters" might be preferable.
３人姉妹です (sannin shimai desu) = (we are) three sisters
４人姉妹の３番目の女の子 (yonin shimai no sanbanme no onna no ko) = the third girl of four sisters
ブラザーズ (burazāzu) or ブラザース (burazāsu) would probably be understood, but these words are usually used when writing English company names or names of movies among other things in katakana.