sister vs little sister

sister and little sister both are nouns.

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

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

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

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