While I can't comment on the specifics of this case, I do know that the al-Husseinis are a very large Middle Eastern clan, that have hundreds, if not thousands of members. Oddly enough, neither the late King Hussein (a Hashemite) nor Saddam Hussein (a peasant) are Husseinis. So its quite possible that Arafat has some ancestral ties to the Grand Mufti's family, but it seems unlikely that the two are closely related.