This isn't a rare!
They are fraternal twins......i have fraternal twins, 1 of my girls has brown curly hair is at present 5ft 3 and wears a size 6 shoe, the other has poker straight hair thats almost blonde is 4ft 11 and wears a size 4 shoe, my girls are like chalk and cheese you really wouldn't believe they were twins, for starters one of my girls is aspergers, and the other is mainstream.
My cousin also has fraternal twins one boy and one girl.
The babies skin colouring is down to the fact that the parents are different races, and that she had 2 eggs fertilized by 2 of his sperm at the same time!
I have to say they are beautiful babies