Ok, Fiat owns nowadays Fiat, Alfa Romeo, Lancia, Ferrari, Maserati, Autobianchi, Iveco and I think Ducatti as well. Hence the cars share a few bits and pieces, like engines gearboxes etc. From time to time Fiat had cooperations with other makes for specific projects, like Alfa 164/Saab 9000/Opel Omega, and MPVs as well. Lately there were discussions to have a very close cooperation with General Motors, have thankfully GM went away, but the current agreement is for GM to use Fiat's small engines (below 2000cc), and Fiat use the GMS's big V6 engines. As an example the 1.9 diesel engine is used by Alfa 147/156/159, Fiat Punto/Grande Punto/Stilo, Lancia, Saab 9-3, Opel/Vauxhall Vectra /Astra... A bit confusing, eh? These days every company has cooperations sharing car bits and pieces and technology as well. There is no single manufacturer not sharing things.
BTW, Porsche is not owned by VW although they always had agreements, like Porsche 924/Audi Coupe, VW Touareg/Porsche Cayenne.
Ford owns Ford, Mazda, Volvo, Jaguar...
Any more info needed???