sister vs sis

sister and sis both are nouns.

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

As nouns, sister and sis are synonyms defined as:

sister and sis: a female person who has the same parents as another person

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