foster-sister vs female person

foster-sister and female person both are nouns.

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

As nouns, female person is a hypernym of foster-sister; that is, female person is a word with a broader meaning than foster-sister:

foster-sister: your foster sister is a female who is not a daughter of your parents but who is raised by your parents
female person: a person who belongs to the sex that can have babies

Other hypernyms of foster-sister include female.

foster-sister (noun)	female person (noun)
your foster sister is a female who is not a daughter of your parents but who is raised by your parents	a person who belongs to the sex that can have babies