A family with no children will work because each boy has 0 sisters and 0 brothers and the girls have 2 times as many brothers as sisters because 2 times 0 is still 0. It also works in a family with 4 boys 3 girls each boy has 3 brothers and 3 sisters and the sisters have 4 brothers and 2 sisters
This is an example of independent events. Rolling a pair of number cubes has nothing to do with and will not affect the outcome of you flipping a coin.