Very cool, love the series and great job on the game!
Garthe is not his brother though. Michael's face was reconstructed by surgeons working for Wilton Knight after being shot. His face was reconstructed in Wilton's image which just so happened to match the face of Wilton's true son Garthe. It's one of the reasons Garthe hates Michael so much.
So, in that context I guess you could call him a brother.
I remember getting the Knight Rider game on NES and to me it was pretty crap, your game blows that out of the water.