My 2-cents. She's not old enough to have gained enough body fat to sustain her through the brooding process. Broody chickens tend to 'grab a quick bite' and lose condition while brooding; this could be fatal to a skinny girl without you helping her by serving HIGH protein and calorie dense foods like sardines packed in oil, meat scraps, fruit, sunflower seeds, mealworms and the like; and of course, only serving these delicacies to her, not the whole flock, to ensure that she's getting enough protein and calories without making the rest of your flock fat!