stepbrother vs half brother

stepbrother and half brother both are nouns.

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

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

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

Other synonyms of stepbrother include half-brother.

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