family line vs sept

family line and sept both are nouns.

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

As nouns, family line and sept are synonyms defined as:

Other synonyms of family line include family, folk, kinfolk, kinsfolk, phratry.

family line (noun)	sept (noun)
people descended from a common ancestor	people descended from a common ancestor