$b$ = number of boys = number of brothers each girl has
$g$ = number of girls = number of sisters each boy has
$b-1$ = number of brothers each boy has
$g-1$ = number of sisters each girl has
$$b-1 = g, \qquad (\text{Equation 1})$$
$$g-1 = \dfrac{b}{2}, \qquad (\text{Equation 2})$$
Plugging in for $g=b-1$ into the second equation:
$$b-1-1 = \dfrac{b}{2} \Longrightarrow b=4, g=3$$