Isn't that assuming that all the children from a set of parents will look very similar? My 3 children all look entirely different, we've got straight blonde hair, dark curly hair and straight darkish hair. Also brown eyes, blue eyes and grey eyes - with all this variation going on between 2 gene donors (if you like) there seems to me to be a large amount of variation in the possible appearance of twin cousins.
I hasten to say that my children all have the same father! lol!