It's possible, its about genectic.
Let "B" for the dark skin allele(dominant and "b" for the white skin allele(recessive) and onli the dominant allele carry express itself.
If their father is a black person who's chromosome carry allele "Bb"
and there mother is white person who's chromosome carry allele "bb"
P Bb x bb
G B b b
F1 Bb bb
(Black) (white)
the black one is sissoko and the white one is gerrard.
It is possible even they have the same father and mother.