If you are careful about it, you should be able to go out. Avoid very crowded places like shopping centres or supermarkets. A little cafe somewhere shouldn't be a problem unless you are sitting near someone with a obvious cold. I took a small bottle of hand sanitiser everywhere I went and cleaned my hands constantly. I also used a tissue to turn on and off taps and tried not to touch toilet seats or the toilet door without a tissue. In my days 10 to 14 I did venture out even to the university where I work which probably in hind sight wasn't a good idea as this I'd consider a crowded place but I was fine. As I said I was really careful of washing and sanitising my hands. I also took extra vitamin C as well as trying to eat lots of fresh fruits and veggies during this week when I was feeling more up to eating.
It will be really great to meet up with your sister and to go out and feel 'normal' again.
Enjoy!
Love Janey xxx