RingoP, very sorry for your pain and the stress it is causing.
All I can share is my own experiences about this. I've been around people that at the end or their life or dealing with a condition they can have visions or knowings, they can too hear the low level energies messing about with them...it can be a blur of things. It can be accurate, it can also not be. It can be a temporarily channeling to then pass without them knowing. It can only just be a full blown psychosis without these suspected elements/ingridiences.
For someone to be posessed you have to - from my understanding, anyone can correct me if you like - have given your permission for it. We have free will. They can frighten us - yes (and using that energy, low levels, I mean), they can't either boost things up from 0 to a 100, they are not that powerful, there are stages. They can not remove our free will. Your grandma is connected to her higher self. Sure too around her age she has her quardian angels and loved ones in spirit connected to her. It can too be that she channeled only she channeled from the low levels which was why such a comment came out. That don't mean she is possessed.
That your grandmother transformed so quickly...I know that stress like moving can make elders very confused, just to use an example. Elders can too suffer TIA attack, but they will pass on, some unnoticed to themselves and loved ones.
Also one has to be careful about what type of medication to give elders. I knew of one who was suppose to have something to calm this individual down - and instead, turned into something of that of your grandma. As we looked into what kind of medication and what it could do to some people it was unbeliavable but we had to fight with a nurse (not even the doctor who had phrescribed it, yet thought she knew everything) who thought she was God or something - but got this individual off the medication.
I am not saying this is what is happening with your grandma, but if you like you could look through the medications. If there is something there that is not working the way it should one should remove it and replace it with something that does. With the individual I mentioned earlier we found something that worked :) made a world of difference.
Anyways it is just an idea, to look into the medication/s, and if you could be there to discuss, book an appointment with a doctor. Make sure you see that doctor. Don't let no home nurse tell you it will go through them.
If you fear black magic (I have had such episode/s myself after finding out that an ex or ex's potential ex? could potentially mix with it), what worked for me is to pray directly to God (who ever God is to you, the highest good) and let God send out the helpers. I think that is stronger than any black magic or hexes or spells. From my understanding black magic, spells are trying to make spirits or energies come together to influence someone else - but again, for what ever the purpose, they can't take over. We got free will and strenght, we even got strenght when we don't think we have (sorry to go on about free will but it is so important).
So sorry too for your dad.
I've prayed for you all.
Please, try to look after yourself too in all this, so you just don't go on automatic. Make sure you rest, eat on routine, have a support system too around you.