You're doing the best you can under the circumstances. There really is only so much you can do. Your daughter is an adult. She should be helping out when she can. Why isn't she helping now that's she's home ? We can't answer that. Have you sit down and discussed the situation with her ? Have you told her how exhausted you are ? Have you asked her to help or did you assume that she would just start helping out ? Some people do need more direction than others.
I would start by sitting down and having a reasonable discussion about the responsibilities she and her children should have now that they are living in your house.
I know some "children" believe that when they go back home to mom, mom will take care of everything for them. Because that's what moms do. Well, you're going to have to tell them that if things keep going they way they are, you won't be there to take care of everything.
You have to take time to take care of yourself. You need a chance to sit and relax too ! Even a simple thing as a bubble bath or hot shower can help reduce some of the stress you're feeling. take 5-10 minutes here or there for your own sanity. take a daily walk. walking is not only great cardiovascular exercise, it's a great way to reduce stress too.
And there really is time to cook healthy meals. It takes a few minutes to chop up some veggies for a mixed green salad. Buy frozen veggies if you're on a budget. You can buy frozen peas, broccoli, kale, spinach, corn, etc... for cheap. Why put one veggie on the plate when you can put two ? a family can eat healthy on a budget. this article should help with some ideas for saving money. www.sparkpeople.com/resource/nutrition_art
Talk to your daughter. set some guidelines for life back at home. take some time for yourself even if it's only a few minutes here and there. You're not Wonder Woman. there really is only so much you can do.