Why doesn't each child look exactly alike?

The reason no child looks exactly alike is because everyone has a two pairs of copies of their genes, and these tend to be different, making it impossible to look exactly alike. Even twins. You can spot at least one difference, whether it be eye color, a mole, birth mark; the things that distinguish another person from another. This applies not only to children, brothers and sisters, but adults too.

Because of different DNA handed down from their ancestors, and different environmental effects.