When we talk about spirits, they are not the souls of the dead, but they are entities of demons!
Yes, the so-called spells, that is, the Wizards, the Mediums, the Spirits, and anyone in the mediums who is in contact with the Demons, can make the people they want sick, thanks to the help of the spirits of the demons who are invoked for such an act!
Thus, spirits can make a person physically ill, but spirits cannot kill people, because God does not allow demons to do so, because they are much more powerful than mankind!
I'm sorry for the death of your grandmother and your father, but I don't think we need to associate what happened with the spirits, as I told you, spirits can make a person physically ill for a certain period, but they can't kill human people, by order of God!
Therefore, you must discover the cause of death of your grandmother and your father, it may be that the fact is due to a genetic family disease, you must know for what disease, and how to put remedies, with the genetic offspring, both of children, that, future grandchildren!
I'm sure they can be family genetic diseases, so the doctors will be able to advise you what to do for possible treatment!
My advice as a Christian is, to pray to God constantly, every day, so that God may give you an answer, and heal you of your illness problems, because where science is limited, God is the Almighty!