Natali, the story of your grandmother's state of mind (would senility be the accurate term here?) reminds me of my late maternal grandfather's condition when he died in December 1998. He was dying at home of progressive bone cancer, and was heavily doped up with morphine to ease his pain--otherwise, his mind was quite good, even at the age of eighty-four.
His death was expected at any day, and the extended family had gathered at his home in Sugar Land, Texas to be near him when it did happen. Christmas was approaching, and the family decided to celebrate Christmas early so as to have one last such event with my grandfather alive, lest he die before or on the actual date itself. I can't remember if it was a day or two before Christmas Eve (the time when my family traditionally celebrates the usual Christmas festivities), but this very sick, non-lucid old man was wheeled out of his bedroom into the den, told that it was Christmas Eve, and sat frail and failing while carols were sung. Apart from the potential needless suffering that could have been avoided by leaving the poor man in his bed, I thought it was wrong (and still do) that he was lied to about the actual date--the family thought they were doing him a favor, when they were actually doing this for themselves. So many "little white lies" that are rationalized as being for the good of the person being deceived are actually for the benefit of the liar.
As it turned out, my grandfather died on December 29th, so he lived to and beyond Christmas after all. And if he hadn't, would that have been so "terrible", for him at least? I just think it's wrong to deceive someone like that, especially someone in such a vulnerable state.