An unpleasant but necessary question: can rats get into the area where she's brooding?
A rat will sneak under a hen at night, and roll the egg away or open it there and then to eat the chick. The hen may gobble up the shell later to keep her nest clean.
I feel it's unlikely a hen who's been sitting for that long would suddenly start the habit, so I'd be looking at vermin.
Sorry this is happening, it's a big problem wherever chickens are kept.
You might be able to move her and eggs at night to somewhere safer, or you might try placing a ratproof cage over the top of her, and putting food and water inside the caged area (as long as the water isn't deep enough to drown chicks).
Best of luck!
cheers
Erica