Fox Studios announces cast for new Fantastic Four movie. The internets loses its frikken mind.

[mathematic]

49. Superpowers are fantasy, human interaction is real

Characters that are human need to act in identifiably human ways. When you race lift one sibling but not the other then either one or both were adopted or they only share one parent. Any of these scenarios impacts the personalities or interactions of these characters so this casting choice creates a conundrum. If you acknowledge that being from a multiracial family involving either adoption, divorce, or parental death impacts a character then you have to fundamentally change decades old characters and if you don't acknowledge that then you are asking for an unreasonable suspension of disbelief from the audience.

On the other hand, if you are not changing the race of the character to match the race of the actor (which you seemed to have assumed), you run into a host of problems trying to explain this to a mass market low/medium-brow comic book movie audience without confusing them.

Look at The Jerk. Some of the comedy comes from the fact that the character doesn't quite get that he's white, not black. The audience knows that there's a difference between a black man and a white man in society, even if the character doesn't.