My Mother needs 24/7 care. I myself take care of her on the weekends. I have a daughter that has a disability and have to care for her. My siblings do not want give up there weekends so they think they can force me by saying if something happens to mom its your fault.
I would suggest they look for weekend care to come in and do what you have been doing. I would give them a deadline to find alternate care. I wouldn't just stop without providing them sufficient time to make other arrangements.
This might be a time to discuss placing your mom somewhere that she might get round the clock care. Depending on her condition, maybe they could arrange for her to stay at a facility on the weekends. For some seniors, sick or dementia patients, that moving back and forth might be too disruptive. Still, you have to do what you have to do to keep her safe.