Al-Husseini is Arafat's last name...
The only time I have read that he is related to the Grand Mufti is when he is speaking of himself or PLO/PA propaganda.
But no matter... we may never know.
You have accurately stated the greater connection between the Mufti and Arafat when you said that his ideological bloodlines are clear and despicable. His actions would do his "uncle" proud.
Right, he's related when it's convenient. His full given name is Mohammed Abdel Rahman Abdel Raouf Arafat al Qudua al Husseini. The fact that he may be related to the mufti (some say uncle, some say cousin, some say distantly, if at all) isn't what gets me the most. I find most disgusting the fact that he's not even native to "Palestine." He's Egyptian.
Good Arafat profile link -
http://www.oslo-war.com/arafat_eng5.htm