If she is receiving care at a facility you may want to speak to them first. However if you do not have decision making authority (and she does not have valid powers of attorney) you will need an attorney to assist you in obtaining guardianship.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
I am sorry that you are in this difficult situation. Obtaining a guardianship can require a lot of time and can cost thousands of dollars. Hopefully you will be able to help your sister get the care that she needs without obtaining a guardianship. For instance, if your sister has the necessary mental capacity, she could execute a power of attorney. A family meeting or mediation may also be helpful.
Please feel free to call my office at 412-221-1116 to discuss the situation in greater detail. I wish you and your sister all the best.
This answer is for informational purposes only. It is not intended to create an attorney-client relationship and may not be relied upon as legal advice. A careful examination of the facts is necessary before a legal answer may be relied on. You should consult your own attorney before taking or refraining from any legal action.