sister vs big sister

sister and big sister both are nouns.

Word	Noun	Adjective	Verb	Adverb
sister	Yes	No	No	No
big sister	Yes	No	No	No

As nouns, big sister is a hyponym of sister; that is, big sister is a word with a more specific, narrower meaning than sister:

Other hyponyms of sister include little sister, half-sister, half sister, stepsister.

sister (noun)	big sister (noun)
a female person who has the same parents as another person	an older sister
a female person who is a fellow member of a sorority or labor union or other group