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