If you include multi-generational not just sibling relationships, you have Rossis, Roberts, and on 4 wheels, Andrettis, Unsers, Pettys, Earnharts and a bunch more, if I stop to think about it -- I am pretty sure that several of the 125 and 250 GP stars are 2nd generation racers as well.
I know that both the Haydons and Bostroms come from flattacker parents -- I believe that both Hayden parents were racers -- I heard that they met at a race, where she kicked his ass. Hard to beat that kinda genes!
You may notice that most of the top new racers started at an extremely young age (some as young as 3-4!!) and that is a lot more likely if the parents are racers. Of course, I am sure some natural talent gets passed too.
That is the excuse I use for being so slow on the track ;-)