Open Question: How to manage time with my mother's cancer...?

My mother was recently diagnosed with breast cancer and has been undergoing chemotherapy. She doesn't have her drivers license and she lives at home (she is a widow as my father passed away 9 years ago) with my older sister. I also have a twin sister, so it's basically the 3 of us who will need to take care of her. I am 24 as well as my twin and my older sister is 38. We all work 9-5 jobs Monday-Friday so it is difficult to be there for her all of the time so she is home alone all day while we are at work. My twin sister wants to make some sort of schedule so that someone will always be there with her which I agree with, however, it will be very difficult for me to keep to it as I work full-time and go to school full-time. I also want to continue to go to try to go to the gym everyday after work (I already lost almost 20lbs and don't want to gain it back) and I want to keep my sanity by going out with my friends at times. My twin sister is not like me and doesn't go out with friends a lot and she doesn't go to the gym so I know she has a different outlook. I don't want to seem selfish because I really do want to be there for my mom AS MUCH AS possible but how do I balance this??