When hens go broody, they eat and drink significantly less than they would normally do. Whilst this is not an issue for a healthy bird, assuming it sits for 21 days on a clutch of eggs, an extended period of broodiness, such as you describe
can affect health (it can weaken their immune system and leave them more prone to illness as well as causing significant weight loss).
Personally, I'd break their broodiness (put them in a wire crate, slightly elevated off the ground with food and water for 5 days or so) and wait until they become broody again. After they have been broody and sitting in a nest for 3 full days / nights, I'd then give them some fertile eggs.
If you decide to go ahead and get fertile eggs then you also need to have a plan for the excess cock birds that will hatch. Are you happy eating them or selling them / giving them away to be eaten? Is your coop / run space sufficient for additional birds? Do you have a means of separating the broody hen from the other birds, whilst she is brooding (some members separate broody hens for the safety of the chicks). I'm sure there are other considerations to take into account.
These links may be useful
http://www.backyardchickens.com/a/guide-to-letting-broody-hens-hatch-and-raise-chicks
http://www.backyardchickens.com/t/181289/how-to-break-a-broody-hen