Tired of being taken advantage

My 86 year old grandmother has really bad dementia and can’t be left alone so my mom asks me to watch her so she can go to her second job and go to the gym with her boyfriend. I also have a gym membership that she bought me and complains that I don’t use but how am I supposed to go when I’m just a fucking babysitter to go everyone in my family. My sister always makes me babysit her kids. Even when we make plans to hang out I always end up watching them. I love them to death and I would do anything for them but I’m really sick and tired of being taken advantage of from my own family.