quadruplet vs sibling

quadruplet and sibling both are nouns.

Word	Noun	Adjective	Verb	Adverb
quadruplet	Yes	No	No	No
sibling	Yes	No	No	No

As nouns, sibling is a hypernym of quadruplet; that is, sibling is a word with a broader meaning than quadruplet:

quadruplet: one of four children born at the same time from the same pregnancy
sibling: a person's brother or sister

Other hypernyms of quadruplet include sib.

quadruplet (noun)	sibling (noun)
a set of four similar things considered as a unit	a person's brother or sister
one of four children born at the same time from the same pregnancy
the cardinal number that is the sum of three and one