I am so sorry for your loss, Ruby. What a devastating tragedy. I can imagine how you're beating yourself up but you repeatedly warned her that the car was old and now you know there was nothing mechanically wrong with the car. This sounds like a fluke accident that might have occurred regardless of which car she was driving.
I'm sure that doesn't bring you much comfort but please don't blame yourself. I hope you and your family can mourn your loss together and celebrate her memory.