No need to explain anything, other than establish them as brother and sister.
That's literally all I want - a statement of "don't worry guys, they're still brother and sister." I think you misunderstood where my fear is coming from. My fear is that they decided to get rid of the brother/sister dynamic because they didn't want to depict a multiracial family (we already know that they didn't want to hire two black actors in prominent roles so I don't think my fear is unjustified). A statement clarifying that they aren't changing this element would be very reassuring given all the rumors floating around about drastic changes.
Okay, then I agree. I would be very disappointed if the family dynamic was changed. That IS what the Fantastic Four is all about.