Maybe I'm missing the proof provided that they aren't, but story-wise (At least through AC:II) Ezio and Alta?r *have* to be related. There's a scene in AC:II where Desmond controls Alta?r and ends up witnessing the conception of one of Ezio's relatives. Similarly for Ezio to visit Alta?r's memories, following Assassin's Creed lore, they have to be related. :shrug:
No, and no.
Desmond has (outside the animus, in the real world) a flashback to the conception of ALtair's first child. That's the final genetic memory Desmond has of Altair, because Desmond is descended from that child. There is no association between that child and Ezio.
Secondly, Ezio doesn't need to be related to Altair. Only the Animus works through genetic memory. Ezio never uses the animus to view Altair's memories. Rather, what happens is that Altair recorded his memories onto special First civilization artifacts, known in revelations as the Masyaf keys. By activating these artifacts, Ezio can view the memories Altair recorded onto them.