How much should I compensate for my family who won't help care for my grandmother?

We have a set schedule but they won't stick to it? I was my grandmother's care giver for the last 5 years. She was diagnosed with Alzheimers 2 years earlier. I moved back to her home with her and she has just moved into a nursing home 3 months ago. My aunt and my mother bullied me into being the one to move back home. They never listened to me until the last 6 months that I might actually be tired and needed some help once in awhile. By help I mean come over in the evening (after I am home from work) for an hour or two so that I could run for groceries etc. It was an ongoing battle the entire time and it was draining. I just stopped expecting help from them and toughed it out the best way I could. I used alot of vacation hours for grocery shopping! I longed for the days that I would no longer have to beg for their help and vowed that they would never again have this kind of control over me after my grandmother was moved to the home. When she first went in the home, they seemed to be coming everyday for the first week. Unfortunately it only lasted the first week and faded out. I was getting even more tired (if that was even possible at this stage) from going to the nursing home after my regular work day and staying until she was settled into bed at night only to go home and try to get some sleep to do it all over again the next day. I should probably mention that neither my mother or my aunt work! I hold a full time job. I finally had enough and told them that I would come up with a schedule so they could do a couple evenings between them so I could get home some evenings before 9-10pm. I made the schedule so that they both would have a weekend night off and complete days where they didn't have to go in at all. I go EVERY single day, was just trying to make it so I could eat supper before 10pm some nights. It was working somewhat, but everytime they feel a sniffle coming on or they have company over or want to just go out to do whatever they want to do, they opt out and just tell me they "can't" go to the home that night. Guess who it falls back on? Not once in 5 years was I ever absent. No matter what, or how I felt. I felt like I was going to do myself in some days but I just kept going because someone had to get my grandmothers supper and chase her up and down the city streets so she wouldn't wander off and get lost or hurt. They both tell me that she doesn't need someone there all the time with her. Truth is they are just too into themselves to consider how my grandmother might feel. They just worry about their own wants and needs. I don't know if I should do my regular hours and then go back later to do their hours as well or not?? Its not like I would be punishing either of them if I didn't go it would just be punishing my grandmother. So confused. Please any advice?