sibling vs triplet

sibling and triplet both are nouns.

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

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

sibling: a person's brother or sister
triplet: one of three offspring born at the same time from the same pregnancy

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

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