If this is her first time, she may be a little unreliable at it (also leghorns aren't known as great broodies). But it's great that she's sitting.
There are other reasons though why she could be feeling a bit unsettled. If there is any chance rodents might be coming by at night, this could make her more likely to push eggs out (or rats could be trying to take them from under her -- rats will readily do that).
Hens that have mites are also not very good at brooding: the itching etc makes them very restless.
I would make sure she's been dusted, rat-proof the area, and see how she goes if she's still got any eggs left to sit on. Make sure other birds can't get in there to lay or you'll get a mix of hatch dates, and some will be left to die.
If she's basically dumped all the eggs, it's worth trying the other broody if you feel any eggs could be salvaged. Then either break this hen's broodiness by putting her in an airy cage, or sit her on plastic eggs for a while, until you're sure she's settled then slip some fresh fertile eggs under her and away you go.
As long as you check her for weight loss and make sure she's coming out for food and water, she can sit longer than the 3 weeks they normally sit for. But if it goes on for ages broodies can get very weak and thin...
Best of luck with it,
Erica