family line vs descent

family line and descent both are nouns.

Word	Noun	Adjective	Verb	Adverb
family line	Yes	No	No	No
descent	Yes	No	No	No

As nouns, descent is a hypernym of family line; that is, descent is a word with a broader meaning than family line:

family line (noun)	descent (noun)
people descended from a common ancestor	the act of changing your location in a downward direction
	properties attributable to your ancestry
	a movement downward
	the hereditary derivation of an individual
	a downward slope or bend
	the kinship relation between an individual and the individual's progenitors