descent vs family line

descent and family line both are nouns.

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

As nouns, family line is a hyponym of descent; that is, family line is a word with a more specific, narrower meaning than descent:

Other hyponyms of descent include family, folk, kinfolk, kinsfolk, phratry, sept, side, breed line.

descent (noun)	family line (noun)
the act of changing your location in a downward direction	people descended from a common ancestor
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