sibling vs relative

sibling and relative both are nouns.

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

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

Other hypernyms of sibling include relation.

sibling (noun)	relative (noun)
a person's brother or sister	an animal or plant that bears a relationship to another (as related by common descent or by membership in the same genus)
	a person related by blood or marriage

sibling (adjective)	relative (adjective)
	estimated by comparison; not absolute or complete
	properly related in size or degree or other measurable characteristics; usually followed by `to'