family line vs line of descent

family line and line of descent both are nouns.

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

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

Other hypernyms of family line include ancestry, blood, blood line, bloodline, descent, line, lineage, origin, parentage, pedigree, stemma, stock.

family line (noun)	line of descent (noun)
people descended from a common ancestor	the hereditary derivation of an individual
	the kinship relation between an individual and the individual's progenitors