Why would he have to marry his half-sister? He'd just be King when old Henry popped his clogs! Mary would just be married off to some Duke and Elizabeth and Edward would never have been born and Henry should have kept his pecker in his hose... ;) Mind you then we wouldn't have had Elizabethan England and maybe Sir Walter Raleigh wouldn't have discovered baccy or spuds! That would have been a crying shame, no fags or chips.