The mixed race of the parents may have nothing to do with the color of the twins,
"OCA 2 is the most common type of albinism in humans, especially among people of African descent. Characeristics are highly variable, however, ranging from minimal to moderate pigmentation of the hair, skin, and iris. The pigment may be localized in freckles. Individuals of African descent with OCA2 tend to have yellow hair, white skin with localized pigment regions, and irises that are partially or completely pigmented with a tan melanin."
Interesting. I wonder if their respective skin tones will shift as the grow up.
The "paler" twin certainly does not appear to be an albino from the photos.
I hope you are wrong. I've seen this pigmentation problems you reference in some products of dual race relationships, and sometimes those children go through life with an unusual appearance that costs them socially. Both girls look just fine.