half brother vs stepbrother

half brother and stepbrother both are nouns.

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

As nouns, half brother and stepbrother are synonyms defined as:

half brother and stepbrother: a brother who has only one parent in common with you

Other synonyms of half brother include half-brother.

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