I have my own theory of Harry's lineage.  I believe that Harry is a direct descendant of Merlin.  Hence the journals being handed down.  I'm making a guess that there was a child born to Merlin and Morgan Le Fey and the lineage continues through Ebenezer, Margaret, Harry and Thomas. and Maggie.  This would explain how powerful they all have been and the apprehension of the WC regarding the entire family.  The Island in Lake Michigan reminds me of Avalon which was also a place of power in the Arthurian legend.  Just my 2 cents.