Edward is Connors grandfather...
Haytham is Edwards son...
Altair, Ezio, Aveline and the Kenways are different branches in Desmonds family tree that acummulated in Desmond...
There is an in-game e-mail which may be a teaser/easter-egg from Ubisoft Montreal about other branches/ancestors of Desmond based on his processed DNA:
It just seems to convoluted, though. It would be one thing if Edward was a direct descendent of Altair and Ezio. But as it is (at least where I am in the game so far), Edward has absolutely nothing to do with that lineage. He just happened upon an Assassin and stole his shit.
It just seems far too coincidental that he would also turn out to be Desmond's relative, considering how few people in the world are actually Assassins.
It's one thing for Desmond to come from a long line of Assassins. It's another for him to come from multiple lines of Assassins. It's yet another for one of those lines to begin as randomly as Edward's did.