This is true color blind casting. There are siblings of different skin tone and a wide array of ethnicities among the supporting ensemble, which indicates we are to completely suspend disbelief and ignore their race. It does not affect the narrative in this scenario.
Mosher’s approach was color-CONSIOUS casting. By singling out the Dever siblings as African-American in an otherwise Caucasian ensemble, we cannot suspend disbelief, as it would be a specific choice to cast these characters so. It inherently changes the characters’ relationships, which I can understand the Miller estate objecting to.