This is largely a legal question: what are your rights, what would your sister have to demonstrate for you to to lose them, etc.?
What the actual situation on the ground is probably varies by where you are--first by country, then by state.
Lots of states of free to low-cost legal aid groups, but they might only be available to consult on a case that exists, not for consultation, but they might have a website that outlines the process for your. I think the key word might be "conservatorship" in the US, which comes in many flavors.
The other part of it is whether your sister would try to do something like that, and that I do not know..