first cousin vs relative

first cousin and relative both are nouns.

Word	Noun	Adjective	Verb	Adverb
first cousin	Yes	No	No	No
relative	Yes	Yes	No	No

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

Other hypernyms of first cousin include relation.

first cousin (noun)	relative (noun)
the child of your aunt or uncle	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

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