How should I handle a sibling stealing money from my grandparents account?

I really needed to read this article. It’s one of my biggest battles today. I’m new to all this. I am 39 years old and I have been obligated to move in with both of my grandparents because of my suspicion they were being taken advantage of. They are both above 90years old and my grandfather can no longer walk for the most part. Initially my mother took on most of this sort thing however she has recently passed away and it was clear that I was the next suitable person. So I moved into an area of their home that was frequently occupied by random people. It looked like a flop house. And due to my grandparents age and health condition it was clear to me that all these people (that lived there for the most part) (my brother and sister included) were taking advantage. After quickly moving in and kicking everyone out, I was cleaning the room that my sister was living in and I stumbled upon a couple of check books written out by my sister to my sister from my grandfathers checking account, in the amounts of $700.00, $900.00, $450.00, and so on. No amount very small and like I said at least 2 checkbooks. I guess so far in actual cash I'm thinking up and over $21,000.00. I’m so devastated. How could she do this to someone else much less our grandfather who has been the only stable person in our life and would give the shirt off his back to a complete stranger. Please help me decipher thru all of this because I'm literally at a standstill about where do I go from here. This was the last thing I ever thought would or could happen. I definitely need all the help / advice I can get. Thank you!!