I have the same problem with one of my bantam brahmas! My 'Broody Queen' has set herself up in her favorite nest, and fluffs up like a little turkey at anything passing by. She's been there for two weeks with nothing under her, for we have 8 chickens in the coop and 8 chicks in the garage, in other words-max. capacity.
My worry is that she isn't getting enough to eat. I do force her off the nest daily for some outside time and to give her a chance to eat and drink, but she has lost some weight and it looking kinda peaky. Before going broody, she had gained some more weight to hold her over during incubation, but she's not incubating anything at all.... If I leave her there and hope she quits, would she eventually get over herself and stop setting? Or will she keep sitting there, starving herself until she's sick?
I suppose I could give her some hatching eggs, but could she last sitting for another three weeks, when she's already lost so much weight?
The broodies will get off their eggs or empty nests a couple of times a day to eat drink and do their business (which is the most awful smelling poo ever) and then right back at it. They do loose weight during this time and occasionally do starve themselves to death. One of my broodies sat on a batch of eggs that were all bad until they started exploding and I took them away (far far away ugh). Still broody so I then let her have some fresh eggs and she sat them until she got two babies. Then she was done. All told she sat around 8 weeks. She was pretty skinny by the time she came out with the kids. I always give them some corn to help sustain them and it makes their poo not so runny. I also give them broiler grower as it gives them a bit extra to keep going. Of couse you should switch to a chick starter once there are chicks involved. Some girls will give up after a few weeks and some will sit and sit. The fluffing is her way of showing she is hugh and don't mess with her. Forcing her off the nest more often can help break her if that's what you want. Are your chicks too old to pop under her at night? I think the recomended is like 3 days or so, but I integrated a 3 week old chick with a momma once. She did ok but it wasn't the best.
You could also see if there is another BYC member in your area that would like some of their eggs hatched and taken care of for them for a short period of time ( I would expect them to pay for feed though). My momma raised chicks are much stronger and healthier, but a whole lot more flighty and difficult to catch.
Best of luck!