If she isn't broody, she won't sit. Broodiness is determined by hormones and not all breeds are likely to go broody. Even within the so-called broody breeds (those breeds more likely than others to go broody) won't ALL broody and not all the time. I had been keeping chickens for 3 years before I got my first broody hen, despite having some breeds that were more likely to go broody than others.