first cousin vs relation

first cousin and relation both are nouns.

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

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

Other hypernyms of first cousin include relative.

first cousin (noun)	relation (noun)
the child of your aunt or uncle	an abstraction belonging to or characteristic of two entities or parts together
	(usually plural) mutual dealings or connections among persons or groups
	sexual activity between individuals, especially the insertion of a man's penis into a woman's vagina until orgasm and ejaculation occur
	(law) the principle that an act done at a later time is deemed by law to have occurred at an earlier time
	an act of narration
	a person related by blood or marriage