Mom has dementia and is in nursing home. My sister Joann passed away at age 48 and we were weren't sure if we should tell her about it. Sister's husband Anthony finally told mom that Joann died and was cremated. Mom cried about it then asked if she could see her now. She didn't understand that this happened several months ago. Yesterday she asked if we had talked to Joann lately because she had a dream that Anthony told her Joann died and was cremated. Should we re-confirm that Joann is gone or just let it go. Its just heartbreaking for us.
That's me . . . And I would tweak my behavior as I needed to depending on mom's response...the objective being to cause her the least pain as possible.
Depends on how bad the dementia is. If it is hurtful to you still, then by all means, don't tell her she died. You can always do so once you are up to it later and it doesn't matter anyway unless she is begging to see your sister and that makes her anxious.