Mentally Incompetant Sister

I have a half sister who is mentally incompetant and now both of her parents are deceased. She lives with another one of my half sisters who is to say not a stable person. My sister who has her states that she has legal guardianship but I dont know how she would have gotten it. I am very afraid for my sister and would like to know how I would go about finding out if my other sister really does have guardianship or not and how I would go about obtaining it. My fahter passed away a year ago and his estate is in probate and all of us are looking to get a lot of money and I dont want my other sister taking advantage of my other sisters money which I know she will, she is a servere drug addict and is supposedly prostituding. Please can anyone give me some advice I fear for the safety of my sister and dont know where to begin