Don't be so quick to dismiss his dream it *MAY*, *MAY* be something.
For example just recently as of the time of this post in May 2020. Little Richie died, well ironically enough 3 days before he died I had the words Little Richie is gonna die, flash over my mind. Two or three days before he died(so 5-6 days) the words flashed in my mind. I was thinking of the episode of Family Guy where they pretend to be the boys from the movie with river phoenix, and in the episode fondly remember rock and roll even though it was recently introduced, piano riff whoo.
So it's possible that this is either something in the air a manifestation of sorts or beings are showing you this.
I understand it's been said the Gods try and not to state bad things and always state good things. But it's possible some Gods or whomever is controlling the airwaves is telling you things will occur. I actually find it strange the JoS mentions the Gods don't state anything bad, if anything bad is usually what people ask if things are gonna happen. Not everything is sunshine, I'm sure some people are told to keep their protection and cleaning up and make sure to be cautious to avoid negativity.
Not really sure why negative prophecies aren't mentioned perhaps because negativity can so easily be dealt with that it's better not to worry than to worry. But anyways don't dismiss the dream it sounds awful losing a grandparent but what can you do.
Maybe without telling your family anything secretive i.e. JoS stuff. Mention your dream and gauge their reactions, at least maybe it's a good way to have a conversation with your grandmother and see what she thinks. Obviously I wouldn't be surprised if the jebus mumbo jumbo occurs assuming she is a xtian. But non the less you never know where the conversation leads to just keep the JoS stuff out.