big brother vs blood brother

big brother and blood brother both are nouns.

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

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

Other hypernyms of big brother include brother.

big brother (noun)	blood brother (noun)
an older brother	a male with the same parents as someone else
	a male sworn (usually by a ceremony involving the mingling of blood) to treat another as his brother