Me and my sister are both mixed race but pass for white, i have dark curly hair and my sister is blonde and straight haired. My dad is a black man, mum is blonde and white, had dna tests and everything - definitely related to my dad and not the postmans kids, mum just has REALLY DETERMINED genes. Genetics is weird yo
It can get even weirder when both parents are mixed heritage. A few years ago, fraternal twin girls were born to mixed heritage parents and while the resemblance was undeniable, one had dark skin and hair, the other was pale and fair. Fraternals from the same birth!
I suspect that might even be the case here.
Edit, here's an example I found:
http://www.dailymail.co.uk/femail/article-2977470/One-million-mixed-race-twins-reveal-prove-sisters.html