I was surprised to see how many of us have mothers we have to take care of. Sammies, my mom often needs help and instead of reaching out to others for assistance, she just lets me swoop in and save the day for her. She even expects it and acts as though I'm neglecting her if I'm not willing to sacrafice of myself to help her. And I would feel guilty if I didn't! It took a very long time for me to realize that this was a pattern and that I was letting my mother let me take of her. It's hard to say no to your mom because she brought you into this world and took care of you herself, but you have a disease that you need to deal with and your mom is going to have to step up to the plate and take care of her own obligations. Give yourself permission not to fix her problems and put the responsibility back on her to set up help for herself and your grandmother. She won't change the behavior until she has to, and she doesn't have to as long as you're taking care of things for her.
I hope that didn't sound preachy. I just really feel for you because I think we have the same mom. :) I am constantly having to ask myself if I'm being supportive of my mom, or letting her take advantage of my sense of obligation to fix what needs fixing. I know she doesn't do so intentionally, but it's a pattern she fell into that I had to recognize for her.
Do your best, then to hell with it.