Interesting choice, though if he did get the part, then it's unlikely Williams would be cast as his sister -- or vice-versa. But it's good that they're open to different options rather than defaulting to white.
They could be adopted siblings of course.
I'm still waiting for the announcement that roles like the Black Panther and Luke Cage have been given to a white actor because he had superior acting ability and was better suited for the part.
Wow, no-one has ever made that point before, apart from the hundreds of times I've heard it before.