You are the child not the parent...You sound rather bossy to me...sorry.
Have you ever considered she knows she is over weight? That she sees herself in the mirror every morning? She knows she is not healthy and that you are unhappy with it? How is you behavior helping her mentally?
She may be hiding food just to keep you from griping at her. If she doesn't want to eat what you have in your kitchen she shouldn't have to hide it in her bedroom to keep you from seeing it.
She can't work out very easily with the knee injury. There is very little she will be able to do with weights and treadmills, I know because I have staples in my knees from college softball injuries. I used to run at least 5 miles every night and I played all kinds of sports all through high school. I hate that I can't do the things I used to do and if my daughter was constantly bugging me about it it would make me feel very depressed. I would really like to go out and whack a tennis ball around to get rid of some pent up anger.
She is on numerous medications, do you have any idea how many medications state on the warning label that they may cause weight gain? A lot of them do. If she is taking medications for her Fibromyalgia then she is not going to lose weight. That medication causes weight gain and if the person has depression to begin with it makes it worse.
Her age is also a factor, she is getting older and she can't do as much, even without the botched knee surgery she might have become more sedentary anyway.
Losing weight may not be something she will ever be able to do again. You need to accept her for who she is and continue to have your lifestyle in your home and let her make her own choices about what she eats and things. Making her feel worse abut herself isn't helping.