Wait, so if Karstarks are Orks, are Boltons Dark Eldar, Lannisters Slaanesh worshippers, and Freys Tyranids? Or are Ironborn Dark Eldar?

The Boltons are definitely Dark Eldar, if for no other reason than their obsession with flaying. 

Lannisters…I could see a Great Crusade-era Emperor’s Children working (since Fulgrim is awfully close to Jaime), although some folks have suggested the Dark Angels because of their feud with the Space Wolves (closest stand-in for the Starks, natch). 

Freys…Genestealer Cult, definitely.