Does she puff up and screech when you open the nest box door? When you take her out of the nest, does she just lay flat on the ground? If yes, then she's broody.
If you only have two chickens, then I would get a few more. You can wait for two weeks and then get sexed pullets so you don't get a rooster.
If this is absolutely not an option for you, then you can break her. Try just taking her out of the nest box several times a day. If that doesn't work, you can put a bottle filled with frozen water underneath her to get her up. If she's still broody, then you can put her in broody jail. That's basically a dog crate on some cinder blocks to allow air to get underneath her. Only give her food and water. No bedding. Leave her in there for about three days, then put her back in the coop. If she goes back to the nest box then put her back in the crate for another day. Keep doing this until she doesn't go back into the nest box and she sleeps on the roost at night.