I don't know what I should do, but I don't want my aunt to attend my nan's funeral
That doesn't sound right. Why are you trying to prevent her from attending?
My nan was notorious for being selfish and mean, and she doesn't want her there
Even though it may be unfair to your aunt to not be able to attend, it's still your nan's wishes for her not to go. She can just mourn in her own way at home, still respecting her both ways.
I agree
Now all you have to do is explain this to your aunt. I hope she sees it the way we do.