Have they had a DNA Test?
Don't know, but its certainly concievable (no pun intended) that *fraternal* twins (from two separate ovum) as opposed to *identical* twins (from a single fertilized cell which divided and separated soon after fertilization) could have such different melanin content.
Both the parents are biracial, so it isn’t inconceivable that one of their children would have lighter skin. These are fraternal twins, so the traits for each were a crap shoot.