I can't be called an expert
I can't even imagine how tough this is for you. It may sound trite -- but don't forget to take of yourself during all this. It's what you hear all the time about the sandwich generation -- caught between caring for parents and kids/other family members simultaneously
I know that your mom wants your attention and I understand that -- having been the sick person. And I don't know that any amount of explaining to her or your husband will make them understand the amount of stress it's all putting on you. But wishfullly you can.
Hopefully some of the other people here have something more insightful than I to add here. Just know that there will be times you can't please everyone -- and that doesn't make you a bad person.