Hi, veri, and welcome to TP. I am sorry to hear about the death of your aunt, and of course your dilemma with telling your mum.
I've not yet been through this with my mother (73, moderately advanced Alzheimer's, no short term memory) but know it's coming with some relatives who haven't been in good health. I probably will not tell my mother when it happens; my primary goal with dealing with my mother's dementia is to avoid as much upset, stress, and anxiety as possible.
I do think there's an art form to love lies or white lies. If your mother should ask about her sister, you might say, I haven't heard anything new lately, she was fine the last I saw her, or something similar. I have seen reports here on TP of the person with dementia not remembering someone was dead, being reminded of it, and being incredibly upset on hearing that "news," so my tendency would be to avoid the situation, but whatever works best for you and your mother. When in doubt, I'd go with your gut instinct.
It's not a stupid question or concern; it seems perfectly reasonable to me.
Best wishes to you.