Correct in Chinese.
wrong! it's 兄弟姉妹, quit thinking that you know Chinese so that must be the hanja
Not acknowledged by Naver. It has neither an entry for 兄弟姉妹 nor 姉妹. Naver says 姉 is an abbreviation for 姊, so the canonical form is 兄弟姊妹.
See also: www.duolingo.com/comment/25559686