My mom has been asking to see her mother once-a-day, but her mother has been dead since 1991. Should I tell her the truth?

You may want to read this article: Dad died. Mom has dementia. Should I tell her? https://www.agingcare.com/133806
There's no right answer for everyone, but my first instinct is to tell them once, and after that say things like, "You'll see him soon," and then distract them. It's very hard. I've been there. The artcle will at least show you that your aren't alone.
Carol

Your mom has a different reality right now. Often times it is best to redirect her focus or change the subject. Other times, tell her the truth in a calm and compassionate manner........

llayton2000, I so understand. Usually at night, my Mom gets to thinking about and missing her Mom and does the same. I first acknowledge that she must love and misses her Mom a lot, then always gently remind her that her siblings have all died, and so have her parents. Toward the end of each day, she starts calling each of her sibling's names and verifying with me that they have all died, and she shares little stories about them sometimes, or simply expresses her sense of sadness that they are no longer alive.

I agree with Carol that there is no right answer, however. Some elders become visibly distraught if they are told that someone they love is no longer alive. If that were the case and recurring, I would handle the situation differently.

I do prefer to try to keep Mom aware about her siblings and parents, however. On the other hand, I choose to not add any names of friends and distant family when they die, or become sick, however. She has enough on her mind missing her immediate family without my adding to the roll-call of names of those who have died.

Whatever you may decide to do, good luck in figuring out what may be best for your Mom. If you are still unsure, let her doctor know and ask for medical input on how to handle. Her doctor's reply will take into account her medical and emotional well-being.

I forgot to mention that my grandmother died before I was born.

If it were me and my mom, yes I'd tell her. My mom has asked to see her sister several times the last few months, her sister died from leukemia in 1974, I always tell her that "Audrey passed away in 1974, you remember don't you?" She always says "oh yes".

Hi--I agree with the agingcare expert------Be tactful and yet stern with your reply. If possible, try to follow this up with something that is pleasant. I would not try to make your Mom feel inferior at anytime, as communication can be challening enough for her.
I hope this answres your quesion-as it is my opinion in the matter. Feel free to contact us, with further questions.
Hap

Mom used to ask for my dad all the time and I just told her he was at work. I did tell her once he died and she was very upset . So than when she again I just said he was at work. She does not ask for him anymore. She calls for him some and other people. I just say what? And she tells me what she wants

I think that there is not an easy answer to this... My Mother is on namenda and is declining rapidly... I can imagine her asking for my grandfather in the not too distant future... I never knew him. He died of lung cancer in 1949. I think that you have to respond honestly but then realize they may not remember what you said 5 minutes later. Some times the old theraputic fiblet is the best thing. Also if they realize it can be VERY upsetting...