So, to be "clear" on this: If the roles of both a brother and sister in a show are cast with actors of color, and if they are the only two actors of color in the show, then we're supposed to understand that the CHARACTERS are also supposed to be of color. But if the brother is played by an actor of color and the sister is played by a Caucasian actor, then we are meant to understand that this is truly "color-blind casting," and we are not supposed to "see" the races of the actors at all. Or, at least, we are not supposed to "see" the race of the actor of color, since the character was not written as a person of color. Do I have that right?
I always go back to the great S. Epatha Merkerson's comment on this subject, relative to her role of Lola in the Broadway revival of COME BACK, LITTLE SHEBA. She said at the time that she didn't like the term "color-blind casting" because "it makes me feel like I'm not really there." I thought that was a very cogent comment, and I wonder how she feels about the casting of this new production of ALL MY SONS -- though I don't imagine she'll weigh in on it.