兄弟 names a man's brothers as a set, older and younger together, so it takes a number in front of it. Put in front of a noun it means fraternal or sister, as in 兄弟单位 for a sister organization and 兄弟民族 for the other ethnic groups of one country.
mistakes
Two words share this spelling. This one keeps a full fourth tone, 兄弟, and means brothers. Drop the tone on the second syllable and 兄弟 is the spoken word for a younger brother, or a way of addressing a younger man.
