big brother vs brother

big brother and brother both are nouns.

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

As nouns, brother is a hypernym of big brother; that is, brother is a word with a broader meaning than big brother:

Other hypernyms of big brother include blood brother.

big brother (noun)	brother (noun)
an older brother	a male with the same parents as someone else
	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