foster sister vs foster-sister

foster sister and foster-sister both are nouns.

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

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

foster sister and foster-sister: your foster sister is a female who is not a daughter of your parents but who is raised by your parents

foster sister (noun)	foster-sister (noun)
your foster sister is a female who is not a daughter of your parents but who is raised by your parents	your foster sister is a female who is not a daughter of your parents but who is raised by your parents