I ran into this last year. I had a broody who hatched 1 chick that disappeared a couple days later. No body, no blood. I have no idea what happened to it, it was there at night, then gone in the am. She continued to be broody. I could tell she wasn't doing that great, she'd lost a lot of weight from sitting. I didn't want to risk her sitting on another set of eggs so I went on Craigslist and bought her 4 chicks (incubator hatched). She raised them then when they were big enough to be on their own, she left them. About a month later she went broody again and I broke her in a cage, she was still very thin and hadn't recovered. If I were you, I'd keep an eye on her and if she does continue to be broody you can either buy her chicks to raise, let her sit on more eggs, or break her. I'd consider her health if you're going to let her sit on more eggs. After sitting on a nest for 3 weeks another 3 weeks will be hard on her.