brother vs big brother

brother and big brother both are nouns.

Word	Noun	Adjective	Verb	Adverb
brother	Yes	No	No	No
big brother	Yes	No	No	No

As nouns, big brother is a hyponym of brother; that is, big brother is a word with a more specific, narrower meaning than brother:

Other hyponyms of brother include little brother, half-brother, half brother, stepbrother.

brother (noun)	big brother (noun)
a male with the same parents as someone else	an older brother
a male person who is a fellow member (of a fraternity or religion or other group)
a close friend who accompanies his buddies in their activities
used as a term of address for those male persons engaged in the same movement