Usually they will eventually come down off the nest for a few minutes at a time to eat, drink, and poop. She may be doing it when you're not there. If you're that worried, you can try to put some food and water close to her. When my broody first started sitting, I'd go out there a couple of times a day and offer some food and water (I just held it up for her for a minute or two or until she lost interest). Then when I was finding unmarked eggs in the nest (I marked all the ones she was sitting on at the beginning), I knew she was getting up and didn't worry about it anymore. Their instinct is usually pretty good though, and while they can lose weight, most are not going to starve themselves.