My Best Friend's Brother.

Guys always have that one golden rule, its like an unspoken law, an oath almost. It doesn't matter what one bro does to another bro, as long as he doesn't break the golden rule then all will be forgotten. You never fall for your best friend's sister. You just don't do it, guy rule. But does it work the same for girls? Is there some girly code of conduct saying I can't fall for my best friend's brother? Because if so then I am in some deep shit.