I logged on just to add my two yen to this...
As a person who has experienced hearing such things from an adult, I had to learn to express my feelings to the person who would say such a thing.
An eight year old is no exception, they usually know what they are saying is hurtful, but some don't seem aware of the effect to their words. I would first ask her why she says such a terrible thing, adding the fact that it hurts you to hear it.
If she thinks that CF has a certain lifespan, which she calculates that you are near the age she thinks is "the end", then correcting her in the theory that as long as you take care of yourself, your life is indefinite. Though talking with her mom, to express that you will not accept such behavior (if saying that same phrase to the eight year old doesn't work), and would like to see her mother correct it, seems like another suggestion for it to stop.
I guess having a "come to Jesus talk" with an eight year old would be hard for her to comprehend? (My sister often used that phrase when we had to have a serious discussion with my parents, so I thought it was cute to throw in there.)
I live with my MIL, and she says some pretty nasty stuff, but I can't have any "come to Jesus" type talks, cause she'll just forget it ever happened after an hour or less! (Even if it was a "Come to Buddha" talk, she'd still forget!)
Hope you find a method to handle the situation, nobody likes to hear such crude things, especially from a child who knows better!