I really don't care what colour any of the Four are, but I agree that the Storms should be siblings. Adapted siblings can work too, I suppose - why do we need to adhere to the traditional concept of the homogenous nuclear family?
However, it occurs to me that it would almost be a cliché to make the wise-ass, thrill-seeking Johnny Storm black. We see quite a lot of black characters like that onscreen. Mightn't it be a more interesting and unusual bit of colour-blind casting to cast a black man as the intellectual scientist Reed Richards?
Of course, if Jordan is the man who's tested best for Johnny, then let them go with him.