It mentioned a Haytham Kenway? If so he has to either be a brother, father, or uncle. That sounds about right to me. Ubisoft does not use historical figures as protagonists or relatives of protagonists because real people have real decedents who can really complain about how their family is being portrayed in a game. That is why they don't do that.
Edit: Obviously they put historical people and their families in the game but they try and make historical figures as true to history as possible.