Because they don't look related. I don't give a damn which ethnicity they are in real life, but they're supposed to be brother and sister, so they should look like brother and sister, and they didn't. Don't read anything more into it than that. I wouldn't care if one of them was Hispanic and the other not if they actually looked like they were siblings. Hell, I would've been perfectly happy if they'd cast both of them as black, or Chinese, or Pakistani, or anything, as long as they looked like they were closely related to one another. But they didn't. Not to me.
I find that a little offensive. My wife's siblings have a range of color from dark coffee brown to pale white. My twin children, hispanic/canadian, have two entirely different complexions, neither of which resembles my wife or me. Yet the two of them still call that pinkish crayon skin color despite my best efforts. I have sat them down and put our three arms together to show that we all have different skin tones. My daughter's hair is almost as blonde as the dyed Alba in the movies and her skin is darker than Alba's.
Throughout latin america skin tone varies...a lot and within families.
And Alba's skin tone is so pale that she could be from any European country.
What's more is that in the comics there is nothing to suggest what ethnicity "Storm" is.
Really, Christopher, I agree with a lot of your opinions but as a writer I would expect you to be more observant about the variety of physiology in the world.