foster daughter vs fosterling

foster daughter and fosterling both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster daughter	Yes	No	No	No
fosterling	Yes	No	No	No

As nouns, fosterling is a hypernym of foster daughter; that is, fosterling is a word with a broader meaning than foster daughter:

foster daughter: someone who is raised as a daughter although not related by birth
fosterling: a child who is raised by foster parents

Other hypernyms of foster daughter include foster-child, foster child.

foster daughter (noun)	fosterling (noun)
someone who is raised as a daughter although not related by birth	a child who is raised by foster parents