These may be added, for though not in the direct line, the Hurinionath, the family to which Pelendur and Mardil belonged, were of Numenorean blood hardly less pure than that of the kings, and undoubtedly had some share in the actual blood of Elendil and Anarion.
It is unclear whether that means that they were related to Elendil via his ancestors or actually descended from Elendil, but there does seem to be an implied connection, though not in the direct line (father to son). (page 218, hardcover edition, in Ch. VII, The Heirs of Elendil)
Several pages earlier, there's another version of the same paragraph:
Here follows the roll of the Stewards of Gondor
that ruled the realm and city between the going of Earnur
and the coming of Elessar
The Ruling Stewards of Gondor
The names of these rulers are here added; for though the Hurinionath were not in the direct line of descent from Elendil, they were ultimately of royal origin, and had in any case kept their blood more pure than most other families in the later ages.
But...this is still not canonical. And Faramir says plainly in TTT that
We of my house are not of the line of Elendil. though the blood of Numenor is in us.
It could,however be that Faramir meant that his house is not of the direct, father-to-son line of Elendil (remember that Pelendur rejected Arvedui's claim to the throne of Gondor on behalf of his wife Firiel who was the daughter of Ondoher, the last King of Gondor of the line of Anarion).
Personally,I think it highly unlikely that a Hurin never married at least a younger daughter of one of the Kings,so I felt justified in creating a granddaughter for Elendil who married an ancestor of Faramir's,especially as Tolkien rarely mentioned daughters in his histories of the kings and Stewards.