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