foster brother vs foster-brother

foster brother and foster-brother both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster brother	Yes	No	No	No
foster-brother	Yes	No	No	No

As nouns, foster brother and foster-brother are synonyms defined as:

foster brother and foster-brother: your foster brother is a male who is not a son of your parents but who is raised by your parents

foster brother (noun)	foster-brother (noun)
your foster brother is a male who is not a son of your parents but who is raised by your parents	your foster brother is a male who is not a son of your parents but who is raised by your parents