I think you are over-reacting, although I do understand your concern.
It is unwise and unkind to bring a diabetic a lot of sweet stuff or an alcoholic alcohol, and it is no excuse to say, "But she asked for it herself."
Doubtless she did, but that doesn't make it right to bring her things that are bad for her, and which I have no doubt her GP has forbidden her.
On the other hand, she is 90, and you have not said that she is of unsound mind, so surely she has the right to live the last years of her life as she wants?
I very much doubt that anyone could or should be accused of attempted murder on these grounds.
When you visit, bring her something that you know will do her no harm, and leave the rest of her visitors to make up their minds themselves what they do.