If we're still in speculation mode with little to go on, they could have been two separate families, they could have both been chosen by the crystal, but Noctis is related to both - Izunia from his father and Lucis Caelum from his mother, making him the one true king the gods have been waiting for.
But then "related" makes his plot even darker, he's not ending an unrelated bloodline anymore but his own, plus it'll be even worse to be betrayed by your own family member rather than a stranger.
I think it's almost 100% certain that Izunia was Noct's ancestor. What's up in the air is whether they were related to Ardyn or not.
I like the "not related" interpretation better, it justifies Ardyn's vendetta even more.
But then "related" makes his plot even darker, he's not ending an unrelated bloodline anymore but his own, plus it'll be even worse to be betrayed by your own family member rather than a stranger.