Each cell (other than sperm or eggs) in a person has exactly the same chromosomes:
XX or XY.

To produce sperm or eggs, the chromosomes are split and put into an egg or sperm (an X or Y each).

Question:

Why are not all female and male children of a couple genetically identical?
Why are separate (same-sex) children of the same parents so very obviously phenotypical and genetically different?
Shouldn't they all look the same?