little brother vs blood brother

little brother and blood brother both are nouns.

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

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

Other hypernyms of little brother include brother.

little brother (noun)	blood brother (noun)
a younger 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