sibling vs quadruplet

sibling and quadruplet both are nouns.

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

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

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

Other hyponyms of sibling include half blood, quad, quin, quint, quintuplet, triplet, twin.

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