stepbrother vs blood brother

stepbrother and blood brother both are nouns.

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

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

Other hypernyms of stepbrother include brother.

stepbrother (noun)	blood brother (noun)
a brother who has only one parent in common with you	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