If you think your mother cannot drive, then you need to take her car away from her.
If she kills someone in her car and the relatives of the dead person find out you knew she was a danger and did nothing, then you could be (meaning: could be) held liable.
But it wouldn't be because you are her executor; it would be because you knew she was dangerous and allowed her to drive.
It's a stretch, but people have been successfully sued on less.