How do you handle people that take advantage of you?

I work for an isted living center as a companion to one elderly woman. I do not get paid by the isted living center. I get paid by the elderly woman's family. This is an upscale isted living center and it costs around $2,500 to $3,500 per month to live there. Included in the cost of the monthly rent to live there is meals, a room to live in, housekeeping, and someone to take you to Dr. appointments in a limousine, etc. Well, the question I have is this: Some of the workers have been taking advantage of me. Well, I have let them to some extent, but now I am tired of it. I only come 4 days per week, 3 hours at a time. Whenever "Donna" has an appointment, and they see me. they stop me and ask me to take her to her appointment that same day and yesterday, they even asked me to go get her medications for her. This is not my job and the isted living center is not paying me for her meds or driving her to her appointments. I have tried to work out a deal with them where they drive me and her to her appointments if they give me notice in advance, and one time one of the girls was snooty to me because I did not make an appointment with her to take her to her next appointment. How should I handle this? I am at the point of next time they ask me and do not give me any prior knowledge of her appointments or call me ahead of time, telling them No, I will not take her to her appointment or go with her unless I am giving prior knowledge and that is if I can. I work another job besides this one and I am extremely busy. Not explaining and not getting mad. I do not know how to handle this situation. Any ideas other than mine?