um.. Porsche owns 31 % of VW.. which is to say, controlling interest in VW and therefore Audi, Seat, Bugatti, Lamborghini and Bentley brands
Ford also owns a controlling interest in Mazda (30+ percent) - hence the large crossover of platforms (Focus/Mazda 3, ford Edgae/Mazda CX7/9 etc). Does FORD still own Land Rover too?
GM also owns Cadillac
Fiat - Ferrari, Maserati, Alfa Romeo and Lancia- essentially all of Italy
Cerebus owns Chrysler now which owns the Jeep and Dodge brands
Diamler (of Diamler Benz) controlls Mercedes cars, Freightliner trucks. Until recently, they also had a 30+ percent of Mitsubishi and, well, Chrysler
I am sure Kia, Hyndai, Daewoo are all inter-related, but the specifics escape me.