@pigsquiggle The easiest way to get rid her since she won't die is to move her out of her household and delete her in Manage Households.
Keep in mind that doing that will cause her portrait that appears in the family tree will go blank for any sims related to her. It will be as she never existed.
But since she was immortal and you didn't make her that way, then there was a reason the game did so their is a very small possibility that it could cause problems. But I have done it before and the game just created a new sim to fill the original role. It may have already done that if you moved her into a household some time ago and have been playing her a while.
Another way is If you use mods, then you might be able remove her immortality with MCCC then let her die like other sims. But that doesn't always work and could mess up your save. Again the chances are very small that it could happen but it might.
So I suggest you make a copy of your current saves folder and put it somewhere safe before trying either. That way, you can go back if problems do happen either way.
Hope this helps.