I see that you are thirteen according to your profile. Most hospitals, unfortunately, do not even allow visitors under a certain age (often sixteen or eighteen) without a parent or guardian. If your parents are unwilling to take you, even after you have stated your concerns, and especially if her parents also do not want you visiting, there is nothing you can do. It's frustrating, but sadly, it's the way those policies work.

I would suggest calling her to see how she is after surgery, or bringing her a present or flowers once she is well and out of the hospital. The wait will be frustrating, but she will be able to know how much you care about her.