With alzheimers (had to look up how to spell it) one must remember that the person's mind is not as it used to be. Telling her probably won't help as she can't focus on the present. Depending on which stage she's in. I know most tend to think it's the past. I don't know if you get along w/ her or not. If you do, you could limit your time, just being there to bring you both comfort. If you don't get along w/ her, then don't see her. Don't put yourself in the situation where you will be triggered. I know that sounds harsh. But w/ alz. esp. the person can't control what they're saying.
Do what you think is best for you. Maybe try a support group for alz. family members either locally or online & see how others handle dealing w/ this.
Best of luck.
Love,
RhysMadison