It sounds to me as if you and DD are both very stressed, goodgran. Hugs! It also seems as if you and DH are very conflicted, you want to continue helping for the GC's sake, but you are fed up w/ DD's expectations and want to stop. I agree w/ those who say you need to sort out what you are and aren't willing to continue to do. Also, I agree that change should come gradually, so you all have time to adjust, and so that DD doesn't feel as if she had the rug pulled out from under her. I hope this will all be resolved over time.
DD may be more grateful for your help than you think, just doesn't take the time to show it, especially when she's upset about something else. You now know she doesn't show much gratitude, so please let go of that expectation. At the same time, please don't let her frequent complaints get you down, they are probably more a reflection of her frustrations than anything you've done or haven't done. I'm another one wondering what she blew up about though. Maybe it's key to some of the tensions between you.
Meanwhile, kudos for being there for your GC! Also, I'm so sorry about your and DH's health issues. Please remember to take care of yourselves even if it means saying no to a few things. Hopefully, DD can understand that if you don't take care of yourselves, you won't be able to do much for her and the GC, anyhow. Besides, you deserve to have some time and space for self-care and relaxation.