Genetics is odd! I've seen mixed race siblings who are completely different colours - one light and fairish and the other really quite dark.
Because every child gets half it's genes from each parent and the parents got half from each of their parents and because some genes are dominant and others recessive characteristics can skip a couple of generations and turn up unexpectedly.
My OH was fascinated to attend my family reunion which unites 4 generations of one family and see how similarities may not go straight down the line (grandma to ma to child), but skip around through the family (aunt to niece to cousin) so that my cousin has the same hair type as me, but her sister doesn't!!