Actually you have no guarantee of that, I was just going by percentages. It may well still be the second adopted boy who brings a pet home, but the chances are small. I believe being away from your home for a long period of time means that a child will rush up and do something. What they do is randomly selected, and one option is "Can I have this pet please?" If you adopted a girl first, there is a higher chance they will bring a pet home as they've had a longer time period than the second adopted child to potentially bring home a pet. So basically adopting a girl first heightens your chances of getting that bunny your heart desires, but there is still no guarantee.