Prince Christopher was adopted.
That would explain why his ethnicity doesn't match either of his parents. Minerva is Cinderella's half sister, not her stepsister.
This would explain why the Stepmother and Calliope are white while Minerva and Cinderella are black. Calliope is the older sister; she was the only daughter of the Stepmother's first husband. Then after the Stepmother married Cinderella's father (Calliope was around four years old at this point, while Cinderella was just a baby, having lost her mother in childbirth), she gave birth to Minerva. Even though Cinderella looks younger than Minerva, she's actually about two years older; Minerva is younger than she looks.