Louieandlottie, it shouldn't be hard for your DD to understand that your difficulty with caring for 2 young children is health- related. If she really doesn't get it, maybe you need to sit down with her and spell it out. Sometimes we don't want to face the fact that our parents aren't as strong as they used to be, because it scares us. Maybe that is why your DD wants to ignore the fact that, at the moment, you've only got the energy to cope with one GC at a time.
Nannymarg, if you've felt that you didn't love her since she was born, then presumably it isn't just a case of not liking her current behaviour. I don't honestly know what you can do about that - you can't force yourself to love someone - it is just very sad for both of you. I haven't experienced this myself but one of my SiLs was aware, when very small, that his maternal grandmother loved his brother but not him. It hurt him deeply when he was little but he just shrugs his shoulders about it now. Unfortunately, he never did develop a deep bond with his gran, who died when he was in his teens. Maybe if she had been more honest with herself about her feelings, as you are, things could have been different.
As for the little girl's behaviour, most children seem to go through phases when they are a pain in the neck. My smallest DGD, who is 4, is going through a tantrum phase at the moment! It won't last forever. If I was you, I'd refrain from saying anything to her mum and dad (never seems to make things better!), hang on for the behaviour to change and focus on what you like about your DGD, rather than what you don't. Easier said than done, I know.