Could you refuse to wear a belt?
Absolutely, and I have done so before. Even the most disenfranchised, powerless persons in history still have some basic decisions to make on how exactly they will proceed. Every choice we make has consequences both positive and negative, and will impact what our lives will be like. There are always other things that can be taken away. I want to keep my phone. I want to be able to go to the library. I need transportation to do certain things. I want to be able to leave the house unchaperoned, so I can work/volunteer, because those are important to my future and I want to be able to help people. Some of those privileges my sister has have come as a result of her choice to be yoked and beholden in other fashions.
As children, there is always going to be an inherent power imbalance with our parents. And they have choices to make as well, that will have consequences, but were not necessarily done maliciously. They are going to make decisions they feel are best, the same way we do. So I will continue operating in a way which I will derive maximum benefit for myself and eventually lead me to the life that I want to be living.
a chastity belt and treating depression at the same time doesn't look good for me.
Uncorrelated. I have been manifesting anxiety and OCD for as long as I have been communicative. I will not use it as an excuse for anything, nor does it diminish my capabilities. The reason I communicate so openly about mental illness is that it is possibly the only phenomenon more heavily stigmatized in America than either female sexuality or anti-capital views. It is a medical condition that millions of us have and should be treated as such. You wouldn't suggest that a hay fever sufferer who takes diphenhydramine and experiences fatigue as a side effect be ineligible for chastity. (I also have hay fever.)
the only change her sister might want as a result of wearing the belt is to be free from it and her parents.
That is ultimately what happens for all of us, whether we want it or not. Sooner or later as we grow and mature, we move on with our lives, and they move on with theirs. I know that day is coming when I will be almost entirely independent of their rules, their belts, their oversight. But also their protection and financial support. I am just getting my ducks in a row until then and reminding myself that "This too shall pass."