I asked my stepmom to step back so I can take a picture of her as she goes over the edge of the falls
Did she know how close to the edge she was?
No, she didn't.
It sounds like you wanted to kill your stepmom, which is a very likely outcome of what you did. There is no way she would walk away from a fall like that without at a severe injury at least. This is a morbid and cruel thing to do.
It's called death by misadventure.
You helped her to get into this situation though. She would be fine if you had not told her to take a step back. It is not okay what you did.