"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."
It's possible. My maternal Grandpa has fraternal twin sisters who look very much alike except for their skin color. One is about a shade darker than Condi Rice and the other is 2 shades fairer than Colin Powell. They both have hair as slick as shiny as orientals. Go figure.
There was a white lady named Jackie,
And she went and married a blacky.
The result of the marriage
Was "quads" in a carriage:
Pne black, and one white, and two khaki.