A couple of things @worriedallthetime15
Firstly contact the council for a carers assessment. They may be able to provide some support for you.
Secondly, are you under rheumatology and are you receiving optimal treatment for your pain/health condition? Be the squeaky wheel.
Thirdly, make time to do something for you every day. Something like pilates may be helpful for your core and to take pressure off your joints.
If your gran is living with you, does she get attendance allowance and if not, please apply - it can be used to help with chores. Your mother may also be eligible for it.
In relation to your mother's health, for as long as she has capacity, there is nothing you can do. If she ignores her health and continues eating, that is not your fault. You have agency over yourself, not yiur mother. If she keels over, then she will get medical care and advice. I suspect she has some MH issues too.
If your mother keels over, your grandmother is your responsibility insofar as you negotiate care packages with social services. You cannot and must not do it.
I'm assuming that as your mother is 64, my age, your gran is in her 80s. Your mum must be as stressed about losing her mum as you are about losing yours.
Is your dad on the scene?
I feel for you but you have to put yourself and your needs first to help the others. Please ignore your mum's accusations.
Do you have a job? Would you like one? Something to get you out of the house.