I remember speaking to my doctor about the shots and he did explain some of the side effects but he said it was much better than taking cortisone orally. I wasn't a candidate for surgery so we opted for the shots. The first time it worked immediately and lasted not quite a yr. The second time was over 11 yrs ago and I haven't had sore knees since. The only pain is sometimes when I wake up my one knee is locked and painful but that's where the cartilage had been removed after a skiing accident, has nothing to do with the shots and as soon as I straighten my leg the pain leaves.
I also had one shoulder done more than 13 yrs ago and I still have no problem with it. So for me, I have to say it worked wonders. Probably depends on the pwerson and the severity of the damage. I hope your sister finds some relief for her knees and hope you don't go through the same for many yrs to come.