This is terrible, but enough outrage. Quickly and quietly start looking for another job or a way to start your own business. Keep copies of all correspondance with her about your current position. If she fires you, sue her. You'd win if the firing was over religion. Then she may not have anything to leave anyone if she wants to keep her business up and running.
I know the emotional part of this is devastating, but you have to go into survival mode now. My jw ex-husband tried to hold our home over my head by refusing to sell it, rent it out, or buy me out. He lost all power over me whe I told him he could have it. You should've seen the look on his face. Granted, I lost out on thousands of dollars and lost most of my possessions, but I was free!
Thankfully you won't have to go this route, because although your grandmother is acting badly, at least she's not threatening to kill you. Time and the law is on your side as far as your job is concerned. Imagine the freedom you'd have if you completely got away from her.